The Fish Glossary




    Abdominal cavity

    the part of the body containing the viscera.


    a deviation from the expected (behaviour, colour, form etc.).

    Abyssal zone

    the abyssopelagic layer that contains the very deep benthic communities near the bottom of oceans. At depths of 4000 to 6000 metres, this zone receives no sunlight and so remains in perpetual darkness.


    shaped like a spine or thorn.

    Accessory male

    a male fish which attempts to fertilise the eggs of a breeding female, at the expense of a dominant male.


    the process of slowly introducing fish into a new environment, allowing them to gradually adjust to changes in water parameters, different lighting, temperature etc. This should be carried out with much care and minimal disturbance. The fish will have become used to their environment, so a sudden change in water parameters (hardness, nitrate level, pH, salinity, temperature etc) can result in severe physiological stress to the fish. If carried out too quickly, the immune system will be compromised and the fish will be far more susceptible to pathogens.


    a measure of the hydrogen ion concentration, expressed in terms of pH. Water is comprised of one oxygen molecule and two hydrogen molecules (H20). It is the ratio of two different forms of hydrogen molecule that determine the pH. Pure water has a concentration of positively charged hydrogen ions (H+) exactly equal to that of the negatively charged hydroxide ions (OH−), giving a pH of 7.0 (neutral). The ratio of these two ions determines the acidity or alkalinity; more hydrogen ions result in more acidic water. Acidic water will have a pH value of less than 7.0.


    fish that live in acidic waters.


    lighting which produces high levels of light in the blue end of the spectrum. Blue wavelengths penetrate water far deeper than other visible wavelengths. When used over marine aquaria, these lights promote coral growth and proper photosynthesis in symbiotic algae that corals need to thrive. This type of lighting is usually used in conjunction with bright white lighting, which helps to ‘balance’ the light output for a more pleasing appearance.

    Activated carbon

    a chemical filter media that is used to adsorb dissolved organic matter and other pollutants from the aquarium water. Activated carbon is produced by heating carbonaceous source materials such as coal, coir, nutshells, peat, or wood to a very high temperature. This is known as the ‘activation’ process and the resultant product is a form of carbon peppered with small, low-volume pores that increase the surface area available for adsorption. The length of time for which activated carbon continues to adsorb pollutants varies considerably depending on the level of pollutants present in the water, and it will need to be changed regularly in order to prevent it releasing pollutants back into the water once it is clogged. Activated carbon is used by many (in conjunction with mechanical media) to ‘polish’ the aquarium water, rendering it crystal clear. It can be useful to remove tannins from the water, for example when a new piece of bogwood is placed in the tank. However it should never be used whilst treating the aquarium with any type of medication as it will absorb these and render them ineffective.

    Adipose fin:

    a small fin composed of fatty tissue and usually lacking rays, located between the dorsal and caudal fins of some fish, most notably characins and catfish.


    introduction of air into water, which increases the exchange of oxygen between the water and the surrounding air. Aeration is normally achieved through the use of an external air pump, which provides a constant stream of air bubbles into the aquarium. Aeration occurs when the bubbles burst at the water’s surface, causing agitation of the surface water – very little gas exchange actually occurs whilst the bubbles are submerged in the water column. Many types of filtration - with flow output positioned near or above the water’s surface - agitate and circulate the water in an adequate manner. However, air pumps can be a very handy back-up should the filter fail, and battery powered models are available for emergency aeration during a power cut. Air pumps are particularly useful pieces of equipment for use during hot weather or when using aquarium water treatments, when the oxygen holding capacity of the water is significantly reduced. Many planted tank enthusiasts utilise air pumps on their heavily planted aquaria overnight, as photosynthesis stops during the hours of darkness and the plants respire instead, using up valuable oxygen that any fish may need.


    relating to, involving, or requiring free oxygen.

    Air pump

    a device which supplies air for air-stones, air-curtains, ornaments, skimmers, undergravel filtration and other pieces of equipment in an aquarium. Most modern models use a vibrating diaphragm to force air through a small flexible airline hose. The actual pump sits outside the aquarium, and if it is positioned below water level (for instance in a cabinet beneath the tank) the airline should be fitted with a small non-return valve to prevent water siphoning down the airline in the event of a power cut.


    fish lacking pigmentation (usually white or cream in colour) and with red eyes. Some albino fish occur naturally in the wild, but these are rare; however, many are captive bred on a large scale for the aquarium trade. Albino fish are slightly less hardy than their pigmented counterparts, and are often sensitive to bright lighting. Many cave dwelling species are albino.


    primitive rootless aquatic plants that grow in relation to the amounts of nutrients and sunlight that is available. If allowed to get out of hand, algae can adversely affect water quality by lowering dissolved oxygen and consequently affecting fish populations. A typical aquarium is a perfect environment for algae to grow, as there is usually sufficient light and plenty of nutrients available from fish wastes. Under excellent water conditions, algae should not present too much of a problem and the glass may just need a periodic wipe to remove any light build-up. In the event of a full blown algae outbreak or ‘bloom’, it is best to address the cause rather than resorting to a chemical treatment. Areas to investigate would be nitrate (NO3) and phosphate (PO4) levels, possible overfeeding, hidden fish waste within the substrate, excessive use of plant fertilisers in freshwater tanks, and extreme photoperiods and inappropriate types of lighting. Some types of algae will also flourish where there is insufficient water movement. In freshwater aquaria, fast growing plants will help prevent the growth of algae by out-competing it for available nutrients, but for this to happen, the algae bloom needs to be under control first. There are many forms of algae (some specific to fresh or saltwater tanks) including diatoms/brown slime algae (easily removed), hair algae/blanketweed (grows quickly and can smother plants and décor), black brush algae (again a robust algae that forms black-brown tufts), and blue-green algae (which is actually Cyanobacteria rather than a true algae). Many marine algaes are considered quite desirable (such as red/pink coralline algae) and some types provide a good diet for certain herbivores and can be biologically controlled.


    a measure of the hydrogen ion concentration, expressed in terms of pH. Water is comprised of one oxygen molecule and two hydrogen molecules (H20). It is the ratio of two different forms of hydrogen molecule that determine the pH. Pure water has a concentration of negatively charged hydroxide ions (OH−) exactly equal to that of the positively charged hydrogen ions (H+), giving a pH of 7.0 (neutral). The ratio of these two ions determines the acidity or alkalinity; more hydroxide ions result in more alkaline water. Alkaline water will have a pH value of more than 7.0.


    fish that live in alkaline waters.

    Ambush predator

    a predatory fish that lies in wait for its prey rather than pursuing it.


    fish waste, uneaten food and decaying plant matter will all contribute to the level of ammonia in the aquarium. Ammonia is highly toxic to aquatic life, even at astonishingly low levels, and in mature aquaria the ammonia level should always be zero. In newly set up tanks, ammonia will be present as the tank goes through the maturation process (see Nitrogen cycle). Two forms of ammonia can be found within the aquarium – free ammonia (NH3) and ammonium ions (NH4+). Free ammonia is considerably more toxic than ammonium ions. The proportion of NH3 to NH4+ will depend mainly on pH. At an acidic pH, more of the ammonia will be present as the less toxic ammonium ions (NH4+), while at an alkaline pH, more of the highly toxic NH3 will in abundance. Ammonia poisoning is therefore more common at alkaline pH. In established aquaria with a mature filtration system, both forms of ammonia will be converted by beneficial aerobic bacteria into slightly less toxic nitrite (NO2) well before the ammonia level becomes harmful. Newly set up tanks will not have a large enough population of beneficial bacteria to deal with sharp increases in ammonia, which is why much care must be taken to monitor the water conditions within a new set up in order to prevent easily avoidable losses - a test kit is an invaluable piece of kit in these circumstances. Most test kits will not distinguish between the two types of ammonia, instead giving a reading of total ammonia. There are situations which may result in a dangerous temporary surge in ammonia level, even in mature tanks. These include the following: introducing a large number of fish at the same time, overlooked deaths of sizeable fish, filter failure or lack of maintenance, over-enthusiastic cleaning of biological filter media (this must ALWAYS be carried out in tank water, NEVER in chlorinated tap water), overfeeding, use of certain medications, and power cuts. In such circumstances, the population of bacteria will need time to recover/increase to deal with the unexpected ammonia spike. If fish appear at all unwell, testing for the presence of ammonia and nitrite should be a priority. There are a number of steps that can be taken to bring the ammonia back down to a safe level. These include stopping feeding for a few days, placing a chemical media such as Zeolite into the filter to absorb excess ammonia quickly, dosing the tank with an ammonia removal liquid (be aware that some types will not work with test kits and give an accurate reading, so try to find a product that can be used in conjunction with test kits), and the removal of organic debris from the substrate via gravel cleaning. Do not start thoroughly cleaning the filter media, as this will disturb and remove the necessary beneficial bacteria. Emergency water changes could be risky if you have an acidic aquarium and the fresh water is slightly more alkaline. This would increase the level of toxicity as the free ammonia rises further.

    Ammonia poisoning

    an ammonia spike can prove extremely dangerous to aquatic life and symptoms of poisoning include gasping at the surface, excess mucus production, reddening of the gills, clamping of fins, lethargy, loss of appetite, erratic behaviour, and red streaks on the fins and body.


    creatures able to live on land and in the water, for example mudskippers.


    fish which regularly migrate between fresh water and the sea (or vice versa) at some stage in their life cycle, but not for the purpose of reproduction.


    fish which spend most of their lives in the sea and migrate to fresh water to breed.


    relating to, involving, or requiring an absence of free oxygen.

    Anal fin

    a median ventral unpaired fin, situated between the anus and the caudal fin, which helps the fish to maintain stable equilibrium against rolling.

    Anchor worm

    highly contagious parasites that are not actually true worms, but worm-shaped freshwater copepods. The females bury their anchor-like heads deep into the muscle tissue of the fish (most commonly in areas near the base of fins) and ulcers often form at the attachment point. The male will attach himself to the body of the female who will release egg sacs which the male fertilises. The eggs hatch into free swimming nauplii that will look for new hosts. After releasing and fertilising the eggs, the parents will die, leaving a gaping would that is very susceptible to secondary infection. When infested, the fish will scratch and flash through irritation. Heavy infestations, especially of smaller fish, may lead to hypoxia through increased respiration. The anchor worms can be removed with tweezers, but much care must be taken to ensure the heads do not break off. Aquarium water treatments are also available.


    eel-like in shape.

    Annual species

    fish whose adult life is confined to a single year. Spawning takes place just prior to the onset of the dry season; the adult fish then die, but the eggs survive in the soil, and then hatch at the onset of the rains. This is mostly confined to rivulines/killifish in the order Cyprinodontiformes.


    the lack of oxygen in an environment.


    in front; front (also often used for ‘towards the front end’).

    Aphytal zone

    The deeper portion of a lake bottom that lacks plants.


    the controlled culture of aquatic organisms such as fish, crustaceans, molluscs and aquatic plants, raising them to certain sizes before releasing to the market.


    a sustainable mix of aquaculture (fish cultivation) and hydroponics (growing plants in water) in a symbiotic environment. Fish waste is filtered and used by the plants, and the cleansed water is then recirculated back to the fish.


    the arrangement of plants, rocks, stones, driftwood etc into aesthetically pleasing compositions within the aquarium, along with the necessary equipment needed to maintain such an environment. It is, essentially, underwater landscape gardening. Typically, an aquascape houses small fish/shrimp as well as plants, although it is possible to create an aquascape with plants and décor only.


    rushed calcium carbonate skeletons of reef corals and shells sold as a substrate for marine aquaria. Aragonite is essential for the replication of reef conditions in aquaria, helping to maintain alkalinity and a stable high pH. It is also often used in freshwater rift lake tanks where hard, alkaline conditions are desired.


    an expanse of water with many scattered islands.


    a ring-like coral reef island that nearly or entirely encloses a lagoon.


    the film of algae, bacteria, fungi, and invertebrates that exist on hard substrates in aquatic environments, and often fed on by specialist fish species. Aufwuchs is a term derived from the German for “growth upon”.

    Asexual reproduction

    the mode of reproducing using a single parent, the offspring inheriting the genes of that parent only (effectively creating a clone of the original specimen). Many plants and lower organisms (bacteria, fungi etc) reproduce in this manner.



    water backed up in its course by an obstruction such as the tide or an opposing current. This term is also used to describe a calm body of water (for instance, an inlet or tributary) that is out of the main fast current of a larger body of water, and also for water area off of a main lake that has been separated from its source during the dry season.


    single-celled prokaryotic (lacking a nucleus) micro-organisms, typically a few micrometres in length and occurring in a range of shapes such as rods, spheres, and spirals. Bacteria are present in most habitats such as soil, water, and in the bodies of animals and plants. Many are vital but some are pathogenic. Bacteria are vital in recycling nutrients, with many steps in nutrient cycles depending on these organisms. For example, in an aquarium, several types of beneficial bacteria living inside the filter are vital for the process of breaking down waste matter and dead organic material. They process ammonia and nitrite and release nutrients into the water which can then be used by higher organisms such as algae and plants and these are, in turn, eaten by the fish (and some removed via regular water changes). Bacteria are vital to the health of the aquarium and continued wellbeing of the fish. Water quality is closely related to the bacterial population in an aquarium, which is why water quality must be carefully monitored when setting up a new aquarium where a filter will lack the necessary bacteria to break down harmful waste products. In other instances, bacteria can be pathogenic (disease-causing), which under certain conditions can become a serious threat to fish health. During times of stress, such as after being moved, or if water quality is allowed to diminish, a fish's immune system becomes severely compromised. It is at this point when naturally present bacteria can quickly multiply and cause disease. Poor water quality is usually the chief cause of bacterial disease. However, if water conditions are returned to normal, and the source of any stresses eliminated, there are many effective antibacterial treatments available. A quarantine tank for any new purchases and a master test kit can prove invaluable.


    an electrical device for limiting the amount of current in an electric circuit. A familiar and widely used example is the inductive ballast used in fluorescent aquarium lighting.


    one of the fleshy, whisker-like tactile organs extending from the mouth area of certain fishes, such as catfishes, loaches, and barbs. Barbels are very sensitive organs, and are used to locate food when searching through substrates or amongst vegetation. It is vitally important to use only a fine, rounded substrate when keeping bottom-dwelling fish that possess barbels. Coarse substrates can cause much damage to the delicate barbels, and this in turn can lead to secondary bacterial infections, all of which is so easily avoidable.

    Barrier reef

    a long coral reef running parallel to the shore but separated from it by a lagoon that is too deep for coral growth.


    the land area that is drained by a river and its tributaries.

    Bathyal zone

    the bathypelagic layer is that part of the pelagic zone that extends from a depth of 1000 to 4000 metres (3300 to 13000 feet) below the ocean’s surface. It lies between the mesopelagic (above), and the abyssopelagic (below).Sunlight does not reach this zone, meaning there can be no photosynthesis. Many species such as octopuses, squid, and large whales navigate this zone, but it is difficult for fish to live in.


    substrate formed of unbroken rock strata.


    relating to the bottom of a sea, lake, or river; also pertains to the bottom-dwelling organisms that live in such areas.


    organisms which swim or float just above the sea floor.

    Berlin method

    a means of biological filtration used to maintain a healthy, stable environment within marine reef aquaria, which relies on the use of ample live rock and protein skimming. The surface of the highly porous live rock is covered with aerobic bacteria which convert ammonia into nitrite and then into less-harmful nitrate. With the employment of good water movement, the nitrates permeate deep within the rock where they are then converted by anaerobic bacteria into free nitrogen gas (which leaves the tank). An efficient protein skimmer is used to remove a large portion of the dissolved organic compounds before they break down into ammonia, therefore lessening the biological load on the aquarium. Any residual nitrates are removed via partial water changes. Many aquarists like to supplement calcium with kalkwasser (limewater) on a regular basis. Powerful lighting, such as metal halide or high output T5 fluorescents are needed to keep organisms on the living rock healthy and activated carbon may be used to remove any discolouration in the water. To get any benefit from the living rock you should be looking at using approximately 1kg of rock for every 9 litres of water (this is only a rough guide and more or less rock may be needed depending on the how the tank is aquascaped and the quality of the rock). Try to stack the live rock in a manner that allows for water movement around it, but at the same time it obviously needs to be stable and certain shapes will fit together much more easily than others (some trial and error will be needed). Reef-safe underwater epoxy can also be used to stabilise the hardscape.


    part of a plant or animal that is divided by a deep cleft into two parts e.g. the lobes of a petal.


    resembling a fork; divided or separated into two branches e.g. the subocular spines of some fish that are divided into two prongs.


    a thin, slimy layer on the aquarium glass and décor that is produced and inhabited by bacteria. These perform certain biochemical processes essential to the nitrogen cycle.


    the biological load placed upon an aquarium environment. It is made up of fish wastes, fish food, decaying algae, plants etc. This bioload increases the amount of ammonia and nitrite within the confines of the aquarium and has to be processed by the filtration system.

    Biological filtration

    aquarium filters that use bacteria to break down wastes via the nitrogen cycle, into materials less toxic to fish. There are several different types of filtration for aquarium use, but biological filtration is by far and away the most important with regards to fish health. The idea is to pass oxygenated water through media with a huge surface area. This type of environment is ideal for the species of beneficial bacteria (which process fish waste) to colonise and work efficiently. As an aquarium matures, the bacteria that perform biological filtration multiply in response to the gradual increase in amount of waste being produced in the aquarium - as it is this waste that is their food source. This is why aquariums must be stocked slowly, in order to allow the bacteria to spread and adapt to the increase in biological load. If those bacteria were depleted or destroyed (for instance by cleaning the filter under chlorinated tapwater, prolonged power cuts etc), levels of toxic ammonia would quickly rise and could result in disease and fatalities.


    the region of a habitat associated with a particular ecological community. A biotope may be bound by species, geographical features, or may be only a section of a larger ecosystem (for instance, Lake Malawi which is so vast that it is host to a wide variety of biotopes from rocky shore, the intermediate zones, and open water – and all of which are home to different kinds of aquatic life). In a true biotope aquarium, the fish and plants are all selected by the specific area they originate from, and the decor or aquascape is also designed to represent that area (for example, an Amazon pool biotope, Indonesian peat swamp etc). However, using this terminology in a slightly less strict sense, a biotope aquarium could house fish and plants, which, while although not actually found together in nature, would thrive in the same conditions and can be housed together in a beneficial but more general set-up e.g. fast-flowing stream habitat, riverbank biotope, brackish lagoon etc.


    both parents raising their young.

    Blackwater river

    a waterway with a slow-moving channel that flows through forested lowland swamps and wetlands. Here, the ancient soils have no minerals to increase water hardness, and as surrounding vegetation falls into the water and decays, tannins leach out, rendering the water very acidic and darkly stained (although transparent - very much like the appearance of tea). Blackwater environments are oligotrophic (low in nutrients) compared to whitewater areas, and the unique water parameters have lead to a diverse group of flora and fauna evolving to live in these specialised conditions. Chemically, blackwater rivers are very low in dissolved minerals and often have no measurable water hardness/very low conductivity (similar to that of rainwater). The very acidic, almost sterile water, with a pH between 3.5-6.0, tends to keep parasite and bacterial populations to a minimum.

    Black-spot disease

    the encysted intermediate larval stage of a trematode (fluke) that may be bought in to the aquarium on new fish or snails. It may be found in a fish’s skin (where the larvae burrow underneath scales, causing much irritation), and also in the gills, and eyes. The skin develops brown-black pigmentation (melanin deposits) over the cysts, resulting in the characteristic ~2mm spots. It is unlikely that the trematodes will be able to complete their life cycle in an aquarium situation as this requires the fish to be eaten by a bird or other animal where they will mature inside the intestines of the host and produce huge numbers of eggs. The eggs are then excreted, making their way back into the water, where they develop and the larvae infect the livers of snails, which in turn will be eaten by fish and the cycle continues. Black spot disease is not common and is rarely fatal to aquarium fish, but it can be extremely irritating and rather unsightly. Very occasionally the trematodes may work their way into the eyes, which can cause blindness. As this disease will be unable to complete its full life cycle in the aquarium, there is not really too much that can be done (other than possibly removing all infested snails) but an antibacterial treatment may be necessary if any of the sites of attachment become infected.


    fibrous, green algae of the genera Spirogyra and Cladophora that often form felted sheets in pond environments. Blanketweed floats because of the oxygen bubbles that are generated by daytime photosynthesis having the effect of buoying up the algae so that it rises to the surface. However, during the night, blanketweed consumes oxygen through the continuous process of respiration, and if it is present in large amounts, it can severely deplete oxygen levels overnight, causing major problems for fish. Blanketweed is also sometimes used to refer to similar hairy types of algae in the aquarium. Once it has established itself, blanketweed is notoriously problematical to keep under control, growing alarmingly quickly, taking hold around plants and choking them to death. If your pond or tank experiences a bloom of blanketweed, the first port of call will be to check for excess nitrate and phosphate in the water (the main food sources of the algae) as these are more than likely contributing to the problem. Chemical filter media designed to remove nitrates/phosphates can be used, in addition to manually removing the blanketweed, vacuuming the pond to ensure there is not a large build up of organic matter (sludge) on the bottom, check that you are not overfeeding, keep sensible stocking levels, use rainwater collected in a water butt to top up the pond, and if possible, plant up at least 1/3 of the pond (plants will help to take up excess nutrients, leaving less available for the algae). Ideally the pond will have been situated in a partially shady area that does not receive excessive direct sunlight, and there should be a minimum pool depth of 75cm (2.5ft) to prevent water warming up too rapidly in sunshine, which also speeds up various algae growths. Also be aware that if rainwater is allowed to run over garden soil before it enters the pond, it could increase nutrient levels by leaching fertilisers into the water. There are assortments of other treatments available including barley straw, barley straw extract, liquid algae controllers and so on, but effectiveness will vary considerably depending on a number of factors. The best way to combat blanketweed in the long term is to address the route cause of the problem.


    red chironomid (non-biting) midge larvae, also known as red mosquito larvae, which grows in the sediments of stagnant pools. They are a popular high-protein aquarium fish food, which most carnivores and omnivores find irresistible, usually measuring around 1cm in length. Bloodworms are very useful for conditioning fish prior to breeding, or during a recovery period, and are available in several forms including frozen, live, gel-suspended, or freeze-dried. As this is a very rich food, it should not be fed exclusively, but used 2 or 3 times per week in conjunction with other more complete foods that contain all the vitamins, minerals, and amino acids that a fish needs. As wild bloodworm can come from disease-ridden habitats, it is sensible to purchase products that have either been gamma irradiated (in the case of frozen or freeze dried) or ensure that any live foods come from a trusted supplier that cultures their own worms in a closed, disease-free environment.


    wood preserved under the acidic, anaerobic conditions of bogs that is safe for use in aquaria, providing a natural habitat for many fish and a useful growing medium for some plants. Most varieties of dead wood rot quickly when submerged under water, producing bacterial/fungal blooms and releasing toxins which can be dangerous in the closed system of an aquarium. However, bogwood has spent hundreds of years becoming semi-preserved and waterlogged in peat bogs, which renders it safe to use as it will not rot in a harmful way. Be aware that bogwood will release tannins and humic acid, which will soften the water and turn it an attractive tea colour that is very natural for some acidophilic species - and this will bring out their colours beautifully. Not every aquarist appreciates the discolouration though, and this can be removed via using activated carbon in the filter. Soaking the bogwood in freshwater prior to placing it in an aquarium can sometimes help to remove a major portion of these tannins, but the length of time required is so variable - some pieces will need days, others may require months! Excessive amounts of bogwood should not be used in tanks that contain fish which prefer hard, alkaline conditions (e.g. rift lake cichlids). Many species of suckermouth catfish with specially adapted mouth parts (such as Panaque species) graze on and ingest bogwood, gaining nutrition from the micro-organisms that live inside it - so it can be an essential piece of décor for such aquaria.


    water with a variable salt content between freshwater and full marine (saltwater) conditions. In nature brackish water occurs where rivers meet the sea, and the two bodies of water mix. This estuarine zone often continues for many miles upriver, creating large brackish habitats. In tropical regions, mangroves occur along vast areas of brackish water and are an important habitat for many fish. Brackish water aquaria should be maintained with a specific gravity (S.G.) of between 1.005 and 1.017. Some brackish species prefer an S.G. closer to full marine conditions when adult.

    Branched ray

    a soft or segmented fin ray which divides distally (i.e. away from the point of attachment) into two or more parts.


    relating to the gills.

    Breeding colour

    the intense pigmentation that some species develop during spawning. Also known as spawning colour.

    Breeding season

    the period of a year in which fish are sexually active. Also called spawning season.

    Breeding tank

    an aquarium specifically set up with the purpose of breeding fish. The tank would be free of any predators, and prepared with all the necessary water conditions and physical structures for the species being bred.

    Breeding trap

    a device to prevent the other fishes in the tank (including the mother) from eating new born livebearer fry in an aquarium. The trap is usually a plastic container that can be placed in the same aquarium, with fine slats to allow water circulation, yet not allow the escape of fry into the main tank. There is usually a special chamber for placement of the heavily pregnant mother, and the slats are positioned in such a way that the fry can quickly swim to the safety of an outer chamber and seek refuge from her, at the same time keeping them safe from the attentions of other fish in the community.


    talso known by the scientific name Artemia, brineshrimp are a popular aquarium fish food. The adult brineshrimp are not particularly nutritious unless vitamin enriched, but if enriched with garlic (an appetite stimulant), they can be a great way to get finicky fish to start eating in the aquarium, before moving them on to other, more nutritious foodstuffs. However, baby brineshrimp, or Artemia nauplii are extremely nutritious as they still contain the yolk sac, and are ideal for feeding to fry or tiny species and getting them into great condition. Brineshrimp may be purchased live, frozen, gel-suspended, or freeze dried, but frozen is preferable as it can be acquired with vitamin or garlic enrichment. Baby brineshrimp is sometimes available frozen or in a very handy ‘instant’ form, but it can also be hatched from dormant eggs (cysts) by adding them to warm, aerated, saline water. These should be rinsed to remove salt before offering them to fry. This is actually very easy to do (brineshrimp eggs are often sold to in kit form to children as “Sea Monkeys”!) and can be very cost effective. Most good aquarium stores should stock the cysts.

    Brood hider

    a reproductive guild whereby fish hide their eggs but exhibit no further care for them.

    Bubble nest

    a protective nest at the water’s surface, built by breeding anabantid species, which is composed of bubbles and secretions. The fish take in air from the surface and create small mucus-coated bubbles which stick together and form a foam-like appearance, usually sited between dense vegetation. The sticky bubbles serve as a protective coating for the eggs and the nest is usually attended to and guarded by one or both of the parent fish.


    of or relating to the cheeks or the mouth cavity.

    Buffering capacity

    see Carbonate hardness.


    swild-caught fish species that are caught incidental to the actual target species. Also known as ‘accidental catch’, ‘contaminant’, or ‘incidental catch’, some real gems can be found in this manner, so always check your dealer’s tanks very carefully.



    the secretion of calcium carbonate (CaCo3) by reef-building corals. This is the method used by hard (stony) corals to create their complex skeletal structures.

    Calcium reactor

    a device which maintains the calcium level in a reef aquarium. They are a popular means of replacing the calcium and carbonate taken up by corals in the process of calcification. In its simplest form, a calcium reactor consists of container filled with calcium carbonate (CaCO3) media, over which aquarium water is passed with the addition of carbon dioxide (CO2). Adding carbon dioxide lowers the pH of the water, rendering it acidic and thereby dissolving the calcium carbonate more effectively, in turn providing the aquarium with calcium and alkalinity for uptake by the corals.

    Carbon dioxide (CO2)

    a colourless, odourless gas that is the product of respiration in living organisms, and is also utilised in the process of photosynthesis in plants. When animals – including fish - breathe, oxygen (O2) is used up and CO2 is released. In the aquarium, a gas exchange takes place at the water’s surface; surface agitation causes excess CO2 to be released into the atmosphere and at the same time, O2 enters the water to replace the oxygen that has been depleted. Aquatic plants use up CO2, and produce O2 through photosynthesis. Whilst plants still produce some CO2 as a waste product of respiration, during the photosynthesis process they use up far more than they produce. In an aquarium that is home to only a few aquatic plants, a natural balance of O2 and CO2 is usually established; the plants will obtain enough CO2 from the fish and other organisms, and the overall production of CO2 should never get dangerously high. However, in heavily planted aquaria, where a significant number of plants are growing, the plants will quickly deplete the CO2 in the aquarium within the first few hours of the day (photosynthesis only occurring during daylight hours) and so they must be provided with supplementary CO2 if they are to thrive. Various CO2 injection systems are available to the aquarist, some creating bubbles of CO2 that are ‘injected’ into the water via a fine diffuser (partially absorbing into the water molecules as they make their way up to the surface), and others consisting of a reactor where water is forced downwards through a chamber and CO2 bubbles, which are diffused from the bottom, are trapped by the down-flowing water until they are completely absorbed by the water. These systems are ideal for daytime use, but during the night when plants do not use up CO2, there is likely to be an excess of CO2 within the aquarium. Additionally, plants produce some CO2 at night, meaning that CO2 levels can become dangerously high - causing a lack of oxygen and subsequent breathing difficulties for any fish in the tank. For this reason, it would be wise to either use a CO2 system which can be turned off a few hours before darkness and switched back on the following day, or to introduce additional surface agitation at night (via the use of a decent air pump). The latter will increase the gas exchange at the surface, ensuring that enough O2 enters the water and excess CO2 is allowed to escape.

    Carbonate hardness (KH)

    also referred to as temporary hardness, this is a measure of the carbonates and bicarbonates dissolved in water and represents the buffering capacity of the water i.e. its ability to resist changes in pH. An aquarium with an extremely low KH level will be subject to rapid shifts in pH if it not monitored carefully, causing stressful conditions for the fish.


    a species that consumes primarily the flesh of other animals.


    fish which spend most of their lives in freshwater and migrate to the sea to breed.


    referring to or concerning the tail.

    Caudal fin

    the tail fin, which aids movement.

    Caudal peduncle:

    the (usually) narrow part of the fish's body located between the posterior of the dorsal and anal fins and the base of the caudal fin. c.f.: confer, meaning compare (with). Sometimes used with scientific names to indicate a similarity to the named species but without certain identification.

    Chemical filtration

    the removal of pollutants from the aquarium water by the use of specific filter media designed to adsorb substances that may be harmful to aquatic life. Activated carbon is probably the most commonly used chemical filtration media, but many others are available to suit various needs, such as zeolite, phosphate removers, nitrate removers, and those which remove heavy metals etc. Most chemical media types need periodic replacement or recharging as they will become saturated with pollutants, at which point they will cease to be effective (and some will actually release pollutants back into the water when inundated). Some types of chemical filter sponge will turn a particular colour after aquarium water has passed through it, indicating the particular type of pollutant that was in the tank. Many people like to use such media for preparing rainwater, to ensure it is free of toxins absorbed from the atmosphere, before using in an aquarium.


    derivatives of ammonia where one, two, or three hydrogen atoms have been substituted with chlorine atoms. Chloramines are sometimes used as a bactericide in municipal water supplies. It is toxic to fish but can be neutralised with special products available at good aquatics stores. However, caution is needed, as some products will leave ammonia behind in the water, and this ammonia spike could also have disastrous consequences, depending on pH and the maturity of tank. Unlike chlorine, chloramine will not dissipate from water by itself, and it is being used more and more by water companies as a more aggressive and long-lasting disinfectant treatment. To test for its presence, a Total Chlorine (or Combined Chlorine) test will be required; a Free Chlorine test will not be sufficient as it would give an erroneous reading of zero in the presence of chloramine. Fish affected by chloramine poisoning will show abnormal breathing such as rapid gill movements, gasping etc, and odd swimming behaviour, either rocking from side to side, or remaining fairly motionless on the bottom of the tank. Many aquarists prefer to use Reverse Osmosis (R.O. water) for their tanks, the high quality pre-filters (which receive water at a very slow rate) safeguarding their fish from chloramine poisoning. If in doubt, telephone your local water company and ask if they add chloramine to your supply.


    pigment-containing and light-reflecting dermal cells found in fish, crustaceans, cephalopods, amphibians, reptiles, and bacteria. Mature chromatophores are grouped into subclasses based on their hue under white light: cyanophores (blue), erythrophores (red), iridophores (reflective or iridescent), leucophores (white), melanophores (black/brown), and xanthophores (yellow). Expansion or contraction of a circular muscle surrounding an individual chromatophore effects colour changes by amassing or dispersing the pigment.

    Ciliate scale

    a scale with comb-like teeth along its free edge.

    Clutch tender

    a reproductive guild where the fish look after the eggs once they have been laid.


    water bodies characterised by summer temperatures not exceeding 18°C.

    Coldwater fish

    although generally considered to be fish that do not need a heater, this is perhaps a better description of temperate fish, as an unheated aquarium in a centrally heated British home tends to sit somewhere between 18 and 22oC. True coldwater fish are found at temperatures below 18oC and can withstand winter temperatures of around 2oC, by which time they will have moved down to the slightly warmer deeper regions and will have entered a period of dormancy.


    a disease which results from an infection caused by the aerobic Gram-negative bacterium Flavobacterium (Flexibacter) columnare. The bacteria is prevalent over the warmer summer months and tends to be rife in aquaria where high bioloads exist, where there is stressful overcrowding, where there may be low dissolved oxygen levels, or in systems with general poor water quality; the bacteria usually enter fish through the gills, mouth, or small wounds. The disease is highly contagious and is easily spread through contaminated equipment; for this reason it is a very good idea to use separate equipment for each aquarium when carrying out maintenance to avoid the disease spreading. Columnaris is commonly mistaken for a fungal infection because of the appearance of the external symptoms, grey-white lesions on the body, fins, and gills which can have a ‘fungus-like’ appearance. The lesions typically first appear on the head and caudal fin, and may at first be seen only as paler areas that lack the normal appearance of the rest of the fish, along with some fraying to the fins. In really heavy infestations, the lesions can take the appearance of angry-looking ulcerations - often in the shape of ‘saddles’- with subsequent epidermal loss and obvious build up (and later, necrosis) on the gill filaments, making breathing very laborious. It is not unusual for the fish to produce excess mucus when affected. Treatment must be given promptly, as if left to manifest, the disease can prove fatal within 48-72 hours, especially in younger or weaker fish. Obtain a suitable treatment from your local aquatics store, and add the appropriate dose after carrying out a partial water change and removing any activated carbon from the filtration system. Time is very much of the essence and treatment should be given without haste. To prevent such problems from occurring, ensure you are maintaining high water quality at all times, and that stocking levels remain sensible. The use of a quarantine tank for new purchases is highly recommended.


    the different species of fish kept together within an aquarium.

    Community fish

    a general term used to define those peaceful species that prefer to live within groups of the same species, and get along well with closely-related or other quiet, peaceable species within the same environment. The environment includes the water chemistry, temperature, décor (plants, substrate, hard surfaces etc), current, lighting, and other animals. However, the term ‘community fish’ should really only ever be used as a very loose guide. In the majority of cases, a fish classed as ‘community’ should generally be quite peaceful and will cohabit well with others labelled in the same category. Nonetheless, there will always be exceptions as individual fish do vary in personality, and sometimes different behaviours may be expressed depending on, for example, the number of conspecifics in the aquarium, tank size, layout, and tankmates. Some books and many shops still use labelling systems that class certain fish as ‘community’, but remember this is not infallible as some fish that are, on the whole, peaceable and unproblematic, may squabble with other normally peaceful fish when housed together - yet they cannot be excluded from the community label because in the majority of cases they would not cause any problems. With so many variables, it may not be too surprising then that a combination which works for one aquarist may not work quite as well for another. This is why the term ‘community fish’ should not be relied upon as gospel and should only ever be considered as a rough guide. It is always best to thoroughly research the requirements of any fish prior to purchase; our own Databanks being one of the best resources on the internet.


    the measure of the ability of water to conduct electricity, and indirectly, the measure of dissolved solids (ions) in the water. The SI unit of conductivity is siemens per metre (S/m), although in the aquatics industry micro siemens per centimetre (μS/cm) or milli siemens per centimetre (mS/cm) are more commonly used. This ability of water to pass an electric current is determined by the amount of anions (negatively charged ions) such as chloride, nitrate, phosphate, sulphate etc, and the cations (positively charged ions) such as calcium, magnesium, sodium etc, that are present in the water. The temperature of the water can also affect conductivity, which rises as the temperature increases. Pure water has a very low conductivity (close to zero) and the more dissolved solids present in a sample of water, the more electrical current it is able to conduct and so the greater the conductivity. Conductivity is synonymous with the term Total Dissolved Solids (TDS), which is a measure of all the inorganic solids dissolved in the water and is expressed in parts per million (ppm). Conductivity or TDS are more accurate reflections of the total mineral content of the water than hardness measurements, as ions that do not contribute to hardness, such as sodium, are also measured along with ions such as calcium which do.


    the body of water formed by the junction of two or more streams.


    a member of the same genus.


    belonging to the same species.

    Continuous breeder

    a species that breeds at any time of the year.

    Continuous spawner

    an egg-laying species which spawns repeatedly each day, over a period of a few weeks. This process can be observed in many rainbowfish species.


    any animal that eats coral polyps.


    an infection of the skin, fins, and gills of young fish by the flagellate protozoan Ichthyobodo (formerly Costia) necatrix. Most commonly seen in outdoor pond fish, with stress, poor water quality, and inferior quality foods cited as the main contributing factors. Infected fish can be identified by the appearance of dull blue-grey mucus spots, which in advanced cases, form into a solid coating. Other symptoms include lethargy, respiratory problems, flashing, and fin erosion. In severe cases, the scales and skin may even peel away in strips. Treatment needs to be rapid, and there are many effective medications readily available. It is essential to increase aeration in the pond/aquarium during this time, as the parasites that infect the gills will deprive the fish of oxygen. Salt used to be a recommended treatment, but this parasite is apparently becoming more and more resistant to it. Obviously the cause of the outbreak would need to be addressed quickly – remedy any water quality problems and eliminate any stresses. Keep a close eye out for any secondary infections that may occur in the form of fungus or bacteria getting in to the wounds. The better the water quality, the less chance this will occur.

    Cotton-wool disease

    also known as Mouth Fungus, this disease is caused by Flexibacter bacterium. Most common amongst newly imported freshwater fish or fish that are in poor condition because of deterioration in water quality. Overstocking may also be a contributing factor. At the onset of the disease, off-white marks may appear around the mouth or on the fins and body of the fish. As the infection progresses, cotton-wool-like tufts appear in the mouth region, along with frayed fins and red ulcers on the body (‘scale-less’ fish displaying ulcers with red edges). Most fish that are affected will also exhibit ‘shimmying’ behaviour, and will go off their food, losing weight. Prompt treatment is necessary to prevent the internal organs from becoming infected, and many effective medications are readily available. Remedy any issues with water quality and ensure your aquarium is not overstocked.


    to evade predators, some fish display dark colouration to the top of the body, and lighter colouration underneath. The effect is such that when viewed from above, the fish blends with the dark sea, and when viewed from below, the fish blends with the lighter sky. This countershading has the effect of obscuring the fish from various predators.

    Crenulate scale

    a scale that is intermediate in form between a ciliate and a ctenoid.


    active at dusk and dawn.


    reproduction between two distinct, but conspecific, gene pools (e.g. populations of the same species that have evolved separately in different areas). This is not the same as hybridisation.

    Cryptic fish

    these fish are often hard to see because their colouration and/or appendages allow them to blend into the background, or because they bury themselves in the substrate. Cryptic camouflage helps hide these fish from predators.

    Ctenoid scale

    a scale with small spines/teeth on the posterior exposed edge.


    the euthanizing of selected specimens in a group of fry or breeding stock, carried out either to remove undesirable characteristics from the group (e.g. those which are malformed or weak) or to reinforce certain desirable characteristics.


    a projection or point, as on a spine or tooth.

    Cycloid scale

    a round or oval scale with a smooth posterior exposed edge.


    tiny freshwater copepods that can be fed to large fry and small fish. Available in live or frozen forms, but be aware that live Cyclops are capable of attacking small fry.



    also known as ‘water fleas’, this is a nourishing small-sized food for fry or petite fish, and is available in frozen, freeze-dried, or live forms. Daphnia is also fairly easy to cultivate in freshwater ponds, outdoor water containers, or even in a small plastic tank on the windowsill. Clear containers tend to produce the best yield; fill one with mature water from an aquarium or pond and site in an area that receives some sunlight. Use a small air pump to gently aerate the water with large bubbles, stopping it from becoming stagnant (be aware that bubbles which are too fine could become lodged in the carapace of the Daphnia and they could then rise to the surface and die. Keep the bubbles big). Acclimatise a starter culture of Daphnia - available at most good aquatics stores - and feed them with a small amount of baking yeast that you have pre-dissolved in a little warm water. The water in the culture container will turn slightly cloudy when the yeast mix is added, and once the water turns clear again, it means the Daphnia have consumed all the food and it is time to add another small dose of yeast mix to feed them. Do not overfeed as this will foul the water and may kill off your culture – a certain amount of trial and error may be needed, but when food is added to the container, the water should turn slightly hazy and not so cloudy that you can’t see the Daphnia swimming about. Also, avoid using a filter in your culture container because this will remove a lot of the tiny life forms that the Daphnia like to feed on. Maintain water quality in the culture vessel by means of small frequent partial water changes. Some aquarists like to combine harvesting with their water changes, siphoning a percentage of the water from the culture container into a waste bucket through a fine net. The net should then be full of Daphnia and can be fed to your aquarium. An advantage of Daphnia over many other live foods is that they will survive in the aquarium until the fish eat them, and will not foul the water if not eaten straight away.

    Dead spot

    an area of a water body where circulation is minimal and anaerobic conditions develop.


    a chemical additive that removes chlorine or chloramine from water. Chlorinated tap water must always be dechlorinated before using in an aquarium; although chlorine/chloramine is added to tap water to make it safe to drink - killing micro-organisms that would otherwise harm us - it cannot distinguish between such pathogens and those organisms which aquarists treasure (fish, invertebrates, beneficial filter bacteria). Check with your water supplier to see if chloramine is added to your water supply, and if it is, choose a dechlorinator that removes chloramine as well as chlorine, as not all products remove both..

    Deep sand bed

    a biological filtration method used in some marine aquaria. The extra fine sand substrate (grain diameter 0.05-0.2mm) must be around 10-15cm (4-6”) in depth to ensure that a large portion of the sand at the bottom will not be exposed to significant water circulation. This allows anaerobic bacteria to colonise the lower reaches of the substrate, converting nitrate into nitrogen gas, which then dissipates out of the water. An established deep sand bed plays host to a wide variety of organisms living in the aerobic upper layers, such as micro crabs, snails, starfish, copepods, and worms. These creatures burrow and overturn the top 2-3” of the sand bed whilst feeding, and process larger particulate detritus into nitrogenous wastes. This is then moved down deeper into the sandbed by their burrowing behaviour where it is converted to nitrogen gas at the bottom. The gas bubbles that form then rise within the sand bed until they are released into the water, the gas escaping at the surface.


    a device for filtering tap water intended for use in aquaria, by using ion exchange resins.


    dwelling at or near the bottom of a body of water. (Note: when used in reference to fish eggs, demersal means that they sink or are deposited on the substrate).

    Denitrification filter

    an aquarium filter that provides nitrate (NO3) removal via a reductive process whereby the oxygen from the nitrate molecules is utilised by anaerobic bacteria, converting it into nitrogen gas (N2).


    may refer to debris, waste, faecal matter, and disintegrated r particulate material that accumulates in the aquarium and is also often referred to as mulm.


    a measure of the general hardness of water, expressed in degrees (from the German Deutsche harte). 1o dH is equal to 17.9 ppm. 0-4o is considered very soft, 4-8o soft, 8-12o medium, 12-18o fairly hard, and 18-30o is hard-very hard.


    fish which regularly migrate between the sea and fresh water (includes anadromous and catadromous species).


    the condition in which males in a species are of two types, those derived from females by means of sex change and those which are born and remain male.


    the condition in which males in a species are of two types, those derived from females by means of sex change and those which are born and remain male.


    having or exhibiting two colour forms.


    occurring in two distinct forms.

    Dissolved oxygen (DO)

    this is the amount of free oxygen available in water and which is necessary for supporting aquatic life. Oxygen dissolves into water from the atmosphere and from plants/algaes that photosynthesise in the water in the presence of light. Ideal dissolved oxygen levels for fish are approximately 7-9mg/l. Most fish cannot survive levels that are below 3mg/l. Remember that the warmer the water, the less dissolved oxygen there is available to the fish. Do also bear in mind that at night (or on very cloudy days, in the case of ponds), the submerged plants and algae will remove oxygen from the water during respiration. This is why it is a good idea to have some extra means of oxygenation available during these times, such as an appropriately sized air pump; the bubbles will break at the water’s surface and diffuse oxygen into the water. An aquarium or pond with a larger surface area is always going to be more efficiently oxygenated (which is why longer, wider tanks are often better than deeper ones). Always ensure there is some movement at the surface of the water, perhaps provided by a spray bar or venturi. It is the rippling/churning motion at the water’s surface that helps to increase dissolved oxygen. Nitrification in the filtration system also uses up a great deal of dissolved oxygen, another reason to ensure there is enough available at all times. Of course, various species will have different needs and some may show signs of respiratory distress long before others, as they may have a much higher natural dissolved oxygen requirement. For example hillstream loaches and some L-number plecos that have evolved to live in swiftly flowing waters that are absolutely saturated with dissolved oxygen (from the churning currents amongst rapids) will require much higher levels of dissolved oxygen in the home aquarium than for instance a gourami, which comes from relatively still waters and can breathe air from above the water’s surface thanks to its labyrinth organ.


    the condition in which males in a species are of two types, those derived from females by means of sex change and those which are born and remain male.

    Dither fish

    usually a group of small, active, confident shoaling fish kept in an aquarium to encourage other more timid species out into the open. The presence of dither fish signals to other more shy species that there are no predators about and that it is safe to venture out from their hiding places.


    active during the daytime.


    of, on, or relating to the upper side or back of a fish.

    Dorsal fin

    othe unpaired fin(s) on the back of a fish, located along the midline. Most fish have one, but some species do have two, both located along the midline. The main function of the dorsal fin is to tabilise the fish against rolling and to assist in sudden turns.


    a condition caused by bacterial and/or viral infections, nutritional disorders, and which may be exacerbated by poor environmental conditions. The affected fish develop a swollen belly due to the accumulation of fluids in the tissues and body cavities, causing the scales to protrude (giving the fish the appearance of a pine cone). There may also be reddening at the vent, long and pale faecal casts, ulcers on the body, lethargy, gasping, pale gills, colour loss, a pop-eyed appearance (exophthalmia), and the fish will go off its food. As fluid accumulates in the body cavity, the internal organs and blood vessels will become cramped and under pressure. Because of the somewhat numerous and sometimes uncertain causes of this condition, precise treatment can be difficult. Affected fish should be isolated in a hospital tank and given the best possible water conditions and quality foods. Treat with a broad-spectrum antibiotic or a medication formulated for dropsy. Unfortunately, the mortality rate is often quite high. It is thought that the underlying cause of fish becoming infected with dropsy is a compromised immune system that leaves the fish susceptible to infection, which can happen as the result of continued stress from poor water quality, ammonia/nitrite spikes, improper nutrition, overcrowding, transportation stress, or aggressive tankmates to name but a few. Once the affected fish has been moved to a hospital tank and is undergoing treatment, be sure to remedy any possible causes of stress in the main aquarium. Other schools of thought believe that dropsy is an osmoregularity symptom of liver or kidney failure, in which case it may be difficult to remedy.

    Dutch aquarium

    a planted aquarium where the plants are arranged in groups that compliment each other and make up a composition that shows contrast in both colour and shape. The style focuses on neat rows of plants with lower plants in front and taller plants at the back. It is often compared to the way an English formal garden is laid out, and generally speaking there should be no more than one plant species for every 10cm (4”) of tank length used, as this keeps the groups of plants well-defined. Typically, 80% of the floor space should be planted. Diversity between each group of plants can be showcased by the neighbouring species having a variety of different properties such as another colour, height, or leaf structure. This is important to prevent the aquascape looking like a solid wall of indistinct plants taking over the whole length of the aquarium. Spaces or ‘pathways’ (also often referred to as ‘streets’) should be left between each group of plants so as to better emphasise these contrasts and add depth of field.

    Dyed fish

    cruel, unnecessary practice of injecting fish with a dye as a marketing gambit. The fish, often translucent species or albino varieties are painfully injected with garish fluorescent dye and sold on as “disco fish” or “painted fish”. These fish are more susceptible to disease and tend to have a much shorter lifespan than un-dyed specimens. The artificial colours fade quickly.


    of, on, or relating to the upper side or back of a fish.



    a biological community of interacting organisms and their physical environment.


    external parasite which feeds with at least part of its body outside the host e.g. anchor worms, lice, and leeches.

    Egg crate

    a plastic grid that allows water to pass freely through it, and has many uses within the aquarium. It is most commonly pieced together into ‘steps’ in marine tanks, and then used as a rigid base to construct a sloping reef on top with living rock, whilst at the same time allowing water to circulate amongst and underneath the rockwork. Upright pieces can make a handy tank divider, and it may be used horizontally when breeding medium sized fish, as it allows scattered eggs to fall through to safety and away from any predators.

    Egg dummy

    also known as egg spots, these striking round markings are present on the anal fin of many male mouthbrooding cichlids. When spawning, the male fish displays these spots to the female, who then bites at them, believing them to be real eggs. The male releases his sperm simultaneously and she ingests this, fertilising the eggs that are already in her mouth by this point.


    with a slightly concave edge or shallow fork.


    also known as gas bubble disease. Excessive oxygen saturation, as caused by algal bloom photosynthesis or the sudden heating of cold water, can result in embolism. It is caused by excess gas in the blood vessels coming out of solution and forming small bubbles. Large numbers of these bubbles forming, especially in the gill structures, can be fatal. Affected fish may swim upside down or vertically, look as if they are gasping, and may have exophthalmia. Small bubbles may be seen in membranes that are the most gas permeable, such as eyes, skin, fins, and the delicate tissue of the gills.

    Emergent vegetation

    plants growing in water but which are not totally submerged (typically found at the water’s edge). Such plants are ideal for paludariums where the roots are situated in the water but the bulk of the plant grows above it.


    native/restricted to a certain region.


    living within the sediment at the bottom of a body of water.


    an internal parasite.


    the uppermost part of the oceanic zone, from the surface down to a depth of about 200 metres (656 ft). This region is relatively warm and well-lit, allowing photosynthesis to take place. The epipelagic zone is more abundant in marine life that the lower zones.


    a body of water formed where freshwaters from rivers and streams flows into the ocean, mixing with the seawater. Although influenced by the tides, estuaries are protected from the full force of ocean waves and storms by barrier islands, reefs, or fingers of land, mud, or sand that surround them. Fish that inhabit these areas are referred to as estuarine or brackish.


    a term used in taxonomy for the derivation and meaning of a scientific name.


    a reference to waters containing 30 and 40 parts per thousand of dissolved salts; that is in most cases, normal sea water.


    excessive nutrients in an aging aquatic ecosystem such as a lake, (usually caused by the runoff of fertilisers, animal waste, sewage etc) from the land, causes a dense growth of algae/plant life; more decomposing organic matter is then produced by the excessive algae/plant growth than the usual biological processes can overcome, and this in turn depletes the supply of oxygen, leading to the death of animal life within the lake. The quantities of nitrogen, phosphorous, and other biologically useful nutrients are the primary determinants of a lake’s trophic state index (TSI).


    protrusion of the eyeballs.


    still in existence, not extinct.

    External brooder

    a reproductive guild whereby the eggs are transferred to a structure on the fish for incubation, such as a specialised brood pouch.

    External filtration

    external filters, also known as canister filters, sit outside the aquarium, below water level (usually in a cabinet under the tank). Water from the aquarium is drawn into the external canister via a gravity siphon, and this passes through a series of chambers containing different filter media, before it is returned back into the aquarium via a pump situated in the head (top) of the canister. The large size of the external canister in comparison to that of the average internal filter allows for a much greater amount and variety of media to be used, and so provides much more efficient filtration. The fact that the filter sits outside the tank also means that you have more room inside the aquarium itself. External filters tend to be more expensive than internal filters, but they have so many benefits and generally require a lot less maintenance.



    a taxonomic rank in the classification of organisms between genus and order. Organisms belonging to the same family would have evolved from the same ancestors and share relatively common characteristics. A family is comprised of one or more genera.


    capable of producing an abundance of offspring; fertile; prolific.


    the average potential reproductive capacity of a female fish of a given age or size.


    the average potential reproductive capacity of a female fish of a given age or size.


    the process of passing water through various types of media to remove particulate matter and to process nitrogenous wastes, of which an accumulation in the aquarium would be detrimental to its inhabitants. Most filters employ two to three stages of filtration: mechanical, biological, and sometimes chemical. The first stage, the mechanical, filters out the larger, visible particles. Next, the biological stage utilises beneficial bacteria to convert harmful organic pollutants into less harmful substances (ammonia>nitrite>nitrate), and finally chemical media can be used to remove heavy metals, phosphates, nitrates, and other chemical pollutants. Of these three types of filtration, biological is by far and away the most important. Siphoning the substrate at water change time will reduce the mechanical load on the filter, and using a high quality water source (such as Reverse Osmosis water) will reduce the amount of chemical filtration that may be required. It is essential for all fish to be kept with some form of filtration; keeping them without and subjecting them to constant fluctuations in water quality really is unacceptable.

    Fin rot

    a condition in which the fins of the fish deteriorate, caused by several bacteria including Aeromonas, Pseudomonas, and Myxobacteria. The fins will appear split, ragged, or stumpy, and often have a white edge to them. This typically affects fish that are in poor condition, for example those that have been subject to poor water quality, overcrowding, rough handling, poor diet, or fighting/aggression. Secondary infections with cotton-wool disease (Flexibacter) may also arise. If left to progress too far, fatalities may occur, so it is important to promptly rectify any aquariums issues that may have bought about the onset of this condition, and treat with a proprietary fin rot medication. Correct care usually prevents fin rot developing in the first place.

    Fish TB

    also called mycobacteriosis, this disease is caused by the acid-fast bacteria Mycobacterium marinum and M. fortuitum. Affected fish will have an emaciated, hollow-bellied appearance, with loss of appetite and loss of colour. They will also be lethargic and may exhibit exophthalmia (protruding eyes) as well as fin rot, body ulcers, skeletal deformities, and small nodules on the body and eyes. The disease is probably passed on from fish to fish by feeding on infected material, although it can sometimes be passed on from parent to offspring, especially in livebearing species. Some species are also more prone to Fish TB than others, for example, many of the labyrinth fishes. As with many diseases, apparently healthy fish may harbour the infection without ever showing any signs of the disease; however, an outbreak may occur if fish carrying the infection are subjected to poor environmental conditions. Isolate infected fish and try treating them with a course of suitable antibiotics, but be aware that severe cases may be beyond treatment. Following an outbreak, ensure that all equipment is thoroughly disinfected. ***Important note*** Fish TB is a zoonotic disease, meaning it can be spread from fish to humans - in this case entering via skin abrasions when working in the aquarium. It should be stated however, that Fish TB (caused by M. marinum/fortuitum) is not the same as tuberculosis (which is caused by M. tuberculosis). A person infected with Fish TB would exhibit skin granulomas, particularly in areas where there may be cuts on the skin and which have been submersed in the aquarium water e.g. fingers, hands, forearms. It can manifest quite slowly over a period of some weeks and during this time the lesions will grow and spread and the Mycobacterium marinum may proceed to destroy the soft tissue under the skin. As this disease is not too commonly encountered by doctors, it is extremely important to mention to them that you are an aquarist, as a general oral antibiotic is unlikely to work. It can be successfully treated, but only if the correct combination of drugs are given, sometimes intravenously, and usually over the course of several months. To prevent such an event from occurring, you may wish to wear protective gloves when working on your aquarium, particularly if you have any abrasions on your skin (even a simple small paper cut or torn cuticle is enough for M. marinum/M. fortuitum to enter the system!). Always use a siphon starter bulb when siphoning water from the aquarium during maintenance, as this disease can be caught when accidentally ingesting water from the siphon. And of course, prevention is always better than cure – ensure that any new fish purchased undergo a period of quarantine before adding to the main aquarium, and make certain you provide excellent environmental conditions at all times.

    Fishless cycling

    a newly set-up aquarium will have no waste processing bacteria living as yet in the filter media, the essential bacteria only building up once a source of waste is introduced (conventionally with the addition of the first fish to the tank). If fish are introduced straight away into a system devoid of beneficial bacteria, they will be subjected to large fluctuations in water chemistry as the filter matures. This period of maturation can be very harmful to the fish if it is not carefully and correctly managed. Fishless cycling is a more humane method that can be used to mature the filter media and ensure a stable environment before any fish are introduced. It is achieved by the regular addition of a small amount of suitable ammonia-based waste product (available at most good aquatics stores) to the aquarium water, which is processed by the establishing bacteria. As soon as ammonia and nitrite both drop to zero, the tank is ready for the addition of a few fish. The fishless cycling procedure may take time, but it significantly reduces the stresses and dangers posed to new fish that are introduced to a new aquarium that has not undergone the maturation process.


    a term used to describe when a fish quickly turns on its side and rubs itself against a surface (usually the substrate or tank base). This action gets its name from the quick flash of light seen as the fish turns to rub itself, the light reflecting off the flanks. This behaviour is seen when something is irritating the skin of the fish and they need to scratch/rub to relieve it. It is often associated with ecto-parasites and poor water quality. The occasional flash with no other visible signs may be nothing to worry about (simply a one-off itch) but if it is occurring frequently, there is a good chance that it indicates a problem such as the beginnings of whitespot disease.

    Fluidised bed filter

    a tall canister that sits outside of the aquarium and contains small heavy sand granules that are used for biological filtration. The dirty water from the aquarium is pumped into the bottom of the filter and is forced up through the sand and then is returned to the aquarium. As this is a fast process, the sand is constantly in motion (being pushed up by the water current and then falling back through the water in response to gravity) and so it appears ‘fluidised’. The flow of water is just fast enough to keep the sand in suspension, and the weight of the sand prevents it from escaping out of the filter. Massive colonies of nitrifying bacteria grow on the surface of the sand and process ammonia and nitrite into nitrate as the water passes through. Unlike canister filters, fluidised bed filters often require a separate pump to be purchased to move the water through it, and many do not come with all the necessary plumbing, so there may be a bit of trial and error needed with set up. This system is not recommended for beginners. Fluidised bed filters require little maintenance other than regular cleaning of the pump so that the water flow/filtration efficiency is not reduced by the build up of algae and debris around the impellor, impellor shaft, intake, and outlet. The sand bed will require frequent monitoring to make sure it remains ‘fluidised’, and be aware that these filters are notorious for not starting up again properly on their own following a power cut. Some fluidised bed filters can be situated underneath the tank, whereas other models are designed to hang on behind the aquarium (in which case you will not be able to site the tank flush against a wall).


    small worm-like external parasites just a few millimetres in length, and known technically as monogenetic trematodes. Various taxa are involved, including Gyrodactylus spp. and Dactylogyrus spp. These flukes often have an obvious hooked attachment organ at the hind end which they anchor into the skin, fins, or gills of the host fish. Fish exhibit flashing, lethargy, frayed and/or clamped fins, excess mucus production, and severe gill infections. Flukes can also cause skin lesions and tissue damage, creating entry sites for secondary infections. It is thought that heavy fluke infestations affecting more than one fish in an aquarium or pond are generally attributed to overcrowding, poor water quality, and stress. Under such circumstances, these parasites can multiply rapidly as they have simple, fast, direct life cycles involving sexual reproduction and only one host. Some monogenetic trematodes are egglayers, and some are livebearers. Prompt treatment with a specific medication is necessary; many that are based on formalin, malachite, or organophosphorous insecticides (which must be used with extreme caution – follow the aquarium treatment manufacturer’s instructions to the letter!) tend to be the most successful. And it goes without saying that any overcrowding, water quality issues or stressors must be addressed.

    Fractional spawner

    a species where the female releases part of an ovulated clutch of eggs at intervals, usually over several days or weeks. This allows the smaller, more immature eggs to be carried in a limited abdominal cavity space as the intervals enable the smaller eggs time to mature; and once shed, eggs mature at different times and thus may avoid a complete loss of a season’s spawning to predation.


    water with a salinity of less than 0.5 parts per thousand.


    a species which feeds on fruit.


    a young fish at the post-larval stage.


    forked, branching.



    one UK imperial gallon is the equivalent of 4.546 litres.

    Gamma irradiation

    a type of high-energy ionizing radiation used to sterilise fish foods, preserving the nutrients and killing any pathogenic microbes that would otherwise destroy them. Gamma irradiation does not alter the way in which the food looks or tastes, nor does it make it radioactive. Many frozen fish foods will have been gamma irradiated, making them the safest option to feed your fish with.

    Ganoid scale

    a non-overlapping or partially-overlapping scale which is of a diamond shape, has a thick outer layer of an enamel-like substance known as ganoine, a central layer of dentine, and a dermal cosmine bony inner layer. Ganoid scales tend to be hard, shiny, and often iridescent.

    Gas bubble disease

    see Embolism.

    Gas exchange

    swater has the ability to carry a certain quantity of dissolved gasses (the amount of which is dependent on a number of factors) and this can have a direct effect upon the aquarium livestoc. Gas exchange takes place at the water’s surface, where gasses are either absorbed from or released into the atmosphere. Several gasses are involved in this process, but the ones that concern us here are oxygen and carbon dioxide. Oxygen is used up by fish during respiration, as well as being utilised by bacteria in the aquarium. This oxygen will then be replaced by gas exchange at the water’s surface from the surrounding air. The amount of oxygen that is transferred will be dependent upon the surface area of the water (a larger surface area will always result in much more efficient gas exchange) and also on the amount of agitation at the water’s surface. Good circulation will increase the efficiency of gas exchange by moving the oxygen-rich water away from the surface and replacing it with water that has much lower oxygen saturation; this lower-oxygen water will then absorb oxygen at the water’s surface. Placing the spray bar return from a filter just above the surface of the water, so that the returning water splashes down onto the water’s surface, is a very useful and easy method for increasing oxygen levels. Powerheads are also very effective for increasing gas exchange, particularly when the flow diverter is angled upwards towards the water’s surface to create a rippling motion. Air pumps may increase gas exchange but only in a negligible way compared to the likes of filter returns or powerheads, as it is only when the bubbles pop at the water’s surface that any gas exchange takes place (nevertheless it must be said that battery powered air pumps can be a lifesaver during powercuts). Other gasses, such as carbon dioxide, are released from the water during gas exchange. Although carbon dioxide is useful for plant growth, for other organisms it is a waste product from the process of respiration, and a build up of too much of it in the water can become dangerous for livestock, causing them respiratory problems. Once again, surface area and agitation of the water’s surface will facilitate the release of carbon dioxide from the aquarium.


    mollusks of the immense class Gastropoda, which is comprised of more than 60000 species including snails, limpets, and whelks (which have a one-piece/univalve shell) and sea slugs (which have no shell). Gastropods are found in both fresh and marine waters.


    plural of genus.

    General hardness (GH)

    also referred to as permanent hardness, this is primarily the measure of the overall concentration of calcium (Ca++) and magnesium (Mg++) ions in the water. Other ions can contribute to the GH but these are usually quite insignificant. The harder the water, the higher the GH. It is often expressed as either degrees hardness (dH) or parts per million of calcium carbonate (CaCO3). One dH is the equivalent of approximately 17.9 ppm CaCO3. Water hardness generally follows these guidelines: 0-4o dH (0-70ppm) = very soft, 4-8o dH (70-140ppm) = soft, 8-12o dH (140-210ppm) = medium, 12-18o dH (210-320ppm) = fairly hard, and 18-30o dH (320-530ppm) = hard-very hard.

    Genital papilla

    a small fleshy tube extending from the urogenital opening at the fish’s vent which is used for depositing eggs or sperm. Many fish that are difficult to sex externally, for example angelfish, can be sexed by the shape of their genital papilla which is extended when the fish are ready to breed.


    a taxonomic rank used in the biological classification of living and fossil organisms. Genus sits below family and above species level and it includes group/s of species with common attributes and which are structurally similar or phylogenetically related. When written down, a genus is always capitalised and italicised.


    paired respiratory organs found in fishes (and some other aquatic creatures) which allow gas exchange to take place between the bloodstream and the surrounding water, for the purpose of respiration. The gill arches contain many delicate gill filaments, which the blood is pumped through. The walls of these filaments are very thin and allow for the diffusion of oxygen from the surrounding water into the bloodstream where it then goes on to reach many of the fish’s vital organs. Some waste products are also released through the gills. Gills are also part of the osmoregulatory system, which maintains the internal balance of salt and water in a fish’s body.


    also known as white mosquito larvae, this is a highly nutritious fish food comprised of larvae of the Chaoborus genus. These larvae are primarily harvested from clear, deep waters where they feed on planktonic crustaceans. As this food is high in protein and unsaturated fatty acids, it is best fed occasionally as a supplement to other foods, and is great for bringing your fish into breeding condition. It is available in frozen form, and sometimes available as live feed. As white mosquito larvae are predatory, the live form should not be used in tanks containing small fry.


    of a spherical shape e.g. pufferfish.

    Glochidial infestation

    a spring and summertime affliction of pond fish, caused by the parasitic larval stage of freshwater mussels. The fish’s fins and/or gills will display grey edges, and closer examination will reveal these are tiny mussel larvae (or glochidia) up to 1mm (0.04”) in size, which have latched on to the fish in order to develop. They are expelled by adult mussels during the spring and summer and they must find a fish host in order to survive and grow. They typically embed themselves in the skin, fins, and gills of the fish and remain parasitic for several months. Eventually fully formed miniature mussels will drop away and begin a free-living existence. While attached to the fish, the glochidia do not usually do a great deal of harm, unless they are present in extremely high numbers. There is no reliable treatment to control the glochidia whilst they are attached to the fish, but this condition rarely warrants medication, provided excellent water quality is maintained to stave off any secondary infections when the glochidia fall away from their hosts. Some aquarists prefer to remove their mussels from the pond during the spring and summer months and house them elsewhere whilst they are shedding their glochidia.


    pertaining to species with distinct sexes, where the male and female reproductive organs are found in separate individuals. This is also known as unisexualism, and the individuals remain the same sex throughout their life cycle. Gonochorism stands in contrast to other reproductive strategies such as hermaphroditism.


    the modified anal fin of male fish, used to transfer sperm to the female e.g. in Poeciliidae (livebearers). It looks like a thin tube and is usually held fairly flat against the body but is moved down and forwards just before copulation.


    a fish that feeds on seeds.


    carrying developing young or eggs.

    Gravid spot

    a dark spot that shows up just behind the anal fin on some female livebearers and which indicates that they are carrying young. The spot will appear more prominent the closer the fish is to giving birth; this is thought to be because the dark eyes of the embryos are much more pronounced at this stage.

    Green water

    this is caused by free-floating, single-celled green algae and can affect both aquariums and ponds; in some cases the water can become so green that the fish are not visible. The most common causes of green water are too much light (particularly direct sunlight hitting the water) and excess nutrients/waste products in the water (usually caused by overstocking and overfeeding). It's a recurring problem in recently constructed ponds, where the water chemistry and planting is still settling down, although older, more established set-ups are not immune. Whilst green water may look rather unsightly, it is not usually toxic for fish if it is dealt with promptly. If an aquarium is affected, it is best to perform a water change and then completely block out the light for 3 days. If live daphnia are available, these can be added to the tank where they will feed upon the microscopic algae, and in turn they will be eaten by the fish. This is why it is important that the aquarium is sited in a place that does not receive too much sunlight, and a pond should always receive partial shade for a reasonable time during the day.

    Grindal worm

    a small, non-parasitic Enchytraeus spp. worm that can be easily cultured as a feed for fry and small fish.

    Grow-out Tank

    a separate aquarium where fry are grown on in a safe environment until they are of a size suitable for sale or for safely moving back into the main aquarium with adult fish.


    a crystalline derivative of purine that is found in the skin of fish, underneath the scales. This enhances light reflection and imparts silvery or iridescent blue/green tones.


    the feeding of nutritious foodstuffs to live prey, so that the nutrients are passed on to the fish for which the prey is intended.


    a form of asexual reproduction related to parthenogenesis (the development of embryos without fertilisation). Once the presence of sperm has activated an egg, it takes no further part in the development of the embryo. Indeed, genetic material from the male is not incorporated into the already diploid or triploid eggs that the mother is carrying (which contain only maternal chromosomes). This results in a mass production of daughter fish, identical clones of the mother.



    a place where a species lives, as defined by parameters such as water temperature, pH/hardness, salinity, dissolved oxygen level, substrate type, velocity, aquatic/riparian vegetation etc, for example: coral reef, ox-bow lake, rainforest swamp, river rapids, tidal estuary.


    relating to the degree of saltiness.


    aquatic fauna and flora living on the sea floor.


    living in the sea.


    a layer where the salinity abruptly changes.

    Hard water

    water with high concentrations of dissolved minerals (calcium and magnesium), usually found in limestone or chalk areas.


    please see General Hardness.

    Head and Lateral Line Erosion (HLLE)

    a chronic disease that affects both freshwater and marine fish, manifesting as lesions on the head that sometimes extend down the lateral line. The lesions are usually progressive, with smaller erosions and ulcerations amalgamating to form larger crater-like lesions, and the fish may become thin and lethargic. The exact cause is unknown, but proposed sources include nutrient deficiency (primarily vitamins A and C), heavy metals, high nitrate, stray voltage, or parasites, and this may be aggravated by stressors such as poor water quality, poor diet, and overcrowding. Secondary bacterial or fungal infections often occur. A range of medications are available, although treatment is not always successful. Recovery also relies on the elimination of potential stressors: where necessary, improve water quality, reduce overcrowding, feed a high quality varied diet, and ensure fish are not being bullied.


    a tributary stream of a river, close to, or forming part of its source.


    a device to warm the aquarium water. A built-in thermostat maintains the temperature at the desired level. Heaters are available as either submersible models that sit inside the aquarium and attach to the glass via suckers, or they may be built into external canister filters (which are ideal for when keeping large, destructive fish).


    spiral shaped.


    feeds on plant material/vegetable matter.


    the same individual possessing both male and female reproductive organs.


    belonging to different species.


    a disease which mainly affects cichlids - particularly Discus - but can occur in a few other species such as carp. It is caused by an internal parasite, the flagellate protozoan Hexamita (also known as Octomitus or Spironucleus). Most cichlids carry low levels of the parasite in their intestines, but under normal circumstances, these will not develop into a harmful infection. However, if the immune system is compromised, for instance due to poor water quality, poor nutrition, or stress, the parasite will multiply and spread. If it gets into other internal organs, the fish soon die. Typical symptoms include darkened colouration, reduced appetite, emaciation, and white stringy faeces. Treatment is via metronidazole based medications in conjunction with environmental improvements.


    a single type specimen upon which the formal description and name of a new species is based.

    Hospital tank

    A separate tank in which sick fish can be isolated from the main aquarium population while undergoing treatment. Also known as a quarantine tank.

    Hospital tank

    A separate tank in which sick fish can be isolated from the main aquarium population while undergoing treatment. Also known as a quarantine tank.

    Humeral process

    a bony projection - typically seen in catfish - extending backwards from the pectoral girdle, immediately above the base of the pectoral fin.

    Humeral spot

    pigment spot located behind the operculum and above the pectoral fin.


    clear, glassy, translucent.


    the offspring resulting from the cross between parents of different species or genera. Hybrids are not always intermediates of their parents, and they tend to have reduced fertility.

    Hybrid vigour

    the improved function of any biological traits in hybrid offspring. Also known as outbreeding enhancement or heterosis.


    a simple and inexpensive instrument that measures the salinity of the water, expressed in units of ppt and Specific Gravity (SG).


    living above, but close to the substrate.


    in an aquarium, an individual fish which dominates the others. The hyperdominant fish could be either sex or any species.


    with a salinity less than that of sea water.


    having a very low oxygen level.



    see Whitespot disease.


    the scientific study of fishes.

    Illicium (plural illicia)

    a long, thin, flexible spine or 'fishing rod' on the dorsal surface of the head, that is derived from the first dorsal fin ray, and which has a fleshy enlargement at the tip (known as the esca) which is used to lure prey within striking distance.


    the electrically-powered propeller that rotates at high speed to move water through filters and pumps.

    Imperial gallon

    equivalent to 4.546 litres or 1.2 US gallons.


    in anatomy, low or lower in position.


    a tributary which flows into another stream or lake.


    microscopic organisms cultured and fed to the smallest of fry. Infusoria grow in water containing decaying vegetation and can be cultivated by crushing a lettuce leaf and leaving in a jar of aged aquarium water in a sunny location. The water will become cloudy, and the resulting infusoria can then be added to the aquarium with a pipette.

    Initial phase

    the first adult colour phase of a sexually dichromatic fish. Also referred to as primary phase.

    Internal filter

    a filter that sits inside the tank and is ideal for smaller aquaria.


    existing or occurring between different species.

    Intertidal zone

    the area that is above water at low tide, and underwater during high tide.


    existing or occurring between individuals of the same species.

    Invasive species

    a species that is not native to a specific location, the introduction and spread of which causes adverse ecological effect.


    an animal which lacks a backbone. For example: cnidarians, crustaceans, and molluscs. Some have a shell or hard exoskeleton.



    behaviour between two fish, which may signal either aggression or intent to breed. When it occurs between two males, it is much more likely to be a trial of strength/territorial dispute. However, if a male and female fish is involved, it may well be pre-spawning behaviour. This type of action warrants careful observation in case intervention is required; it is not unknown for fighting males to cause jaw dislocation. Also known as Lip-locking.

    Junior synonym

    in nomenclature, this is a name which describes the same taxon as a previously published name.


    a young fish which is not yet sexually mature.



    a German word, meaning 'limewater'. This is a highly concentrated solution of calcium hydroxide, which has a very high (alkaline) pH of 12+. It is used to supplement calcium and maintain a high pH level in reef aquaria.


    colour temperature is a term that is often encountered when choosing lighting for specific aquariums. During the 19th century, British physicist William Thompson (Lord Kelvin) devised an absolute thermodynamic temperature scale. During his research, he heated a blackbody radiator which firstly began to glow red, then as the temperature increased it turned yellow, eventually producing a bright blue-white light at the highest temperature. These light colours always follow a pattern and occur at specific temperatures, which is expressed in degrees Kelvin (K). Light is comprised of seven colours, and sunlight (measured at 5500K) emits all seven colours, making the light appear white. Lights with a lower Kelvin rating e.g. 4000K emit a reddish hue (long wavelength/less energy)and those with a higher Kelvin rating e.g. 7500K+appear blue (short wavelength/high energy), meaning that lower colour temperatures indicate a 'warmer' light, and higher colour temperatures equate to a 'cooler' light. To complicate things further, light also changes the further it penetrates down into the water, particularly notable for deeper regions of the ocean where only some colours of light will reach the bottom. This is because when light travels through water, longer wavelength (red) light is absorbed first, with the shorter wavelengths (blue) travelling much deeper and scattering in different directions, making the water appear blue.


    see Carbonate hardness.



    an informal system coined by the aquarium magazine DATZ that is used for classifying Loricariid catfish before they are formally described. See also LDA-number.

    Labyrinth organ

    a specialised accessory respiratory organ found in Anabantoids, which allows the fish to take in oxygen directly from the air. This enables them to live in waters with a low level of dissolved oxygen.


    pertaining to, or inhabiting, lakes.


    a shallow body of water separated from the open ocean by barrier islands, reefs, or sand bars.


    a large, standing body of water that is surrounded by land. Most commonly freshwater, but may be saline.


    spear-shaped; of a narrow oval shape that tapers to a point.


    a large, standing body of water that is surrounded by land. Most commonly freshwater, but may be saline.


    spear-shaped; of a narrow oval shape that tapers to a point.

    Larva (plural larvae)

    a developmental stage between hatching and metamorphosis into the juvenile stage.


    relating to the side.

    Lateral line

    a system of sense organs found in aquatic vertebrates, used to detect movement, pressure gradients, and vibrations in the surrounding water. The lateral line is usually visible as a faint line of pores running lengthwise along each flank, from the vicinity of the gill covers to the caudal fin. In some species, the lateral line may be incomplete, curved, or have accessory branches.


    an ancient and highly porous clay soil that is rich in iron and provides an nutritious medium for growing rooted aquatic plants when it is placed in a layer below the gravel.

    Latin name

    an ancient and highly porous clay soil that is rich in iron and provides an nutritious medium for growing rooted aquatic plants when it is placed in a layer below the gravel.


    an informal system coined by the aquarium magazine Das Aquarium that is used for classifying Loricariid catfish before they are formally described. See also L-number.


    inhabiting still, freshwater (such as natural ponds and swamps).




    having a whitish colouration due to lack of pigment, but normal coloured eyes.


    see Jaw-locking.


    equivalent to 0.22 Imperial gallons or 0.26 US gallons.

    Live rock

    fragmented pieces of calcareous rock (composed of the aragonite skeletons of long-dead corals) that has been naturally colonised by algae, beneficial bacteria, corals, invertebrates, micro-organisms, and sponges that is used in marine aquaria as a biological filter (in addition to providing a natural-looking hardscape). Such rock can be found anywhere in the world's oceans where coral reefs occur, and rocks from different regions have distinct characteristics, from density and porosity to the variety of marine life it supports. Live rock tends to be harvested from areas near to coral reefs where pieces of rock may have broken off from the main reef due to storms/wave action. It may also be aquacultured by seeding small porous rocks in warm ocean water, to be harvested months later.

    Live sand

    natural coral sand from the ocean that is populated with beneficial bacteria and other various micro-organisms, which can be used as a substrate in marine aquaria and also aids in biological filtration.


    a fish that reproduces via internal fertilisation and which gives birth to live young.


    inhabiting running or rapidly-moving freshwater.




    chronic disease of both freshwater and marine fish, caused by an iridovirus, and which may manifest when fish are under stress. Initially, this disease may be mistaken for whitespot, but the small whitish growths on the fins and skin soon clump together to form larger, cauliflower-like nodules. There is no known cure. The best course of action is to provide the fish with a stress-free environment and after a month or two, the nodules should fall off the host fish as they develop their own immunity to the virus. This causes the infected cells to spread into the water, where they may attach to another host via a break in the skin. Although lymphocystis is contagious, it seems to show some host-specificity (only spreading to fish of the same or closely related species), and death from the virus itself is rare. However, secondary bacterial infections are common at the site of the detached nodules, so observe very carefully and medicate if necessary.


    Malawi bloat

    a condition similar to dropsy which affects cichlids from the African lakes. The first symptom is usually loss of appetite, followed quickly by abdominal swelling, increase in respiration, and lethargy. The condition progresses at an alarming rate, and death can occur within 3 days. By the time symptoms are showing, it is usually too late to begin treatment. The cause is currently uncertain, but is most likely thought to be due to inappropriate diet.


    small, evergreen, salt-tolerant trees that grow in oxygen-depleted muddy substrates in coastal saline or brackish waters. The stilt-like roots allow these plants to cope with the daily tides, with most mangroves inundated twice per day. Mangrove forests stabilize the coastline, reducing erosion from currents, storm surges, tides, and waves, and form an attractive habitat for many types of fish seeking shelter from predation.


    saltwater aquarium or environment.


    an area of low-lying land which is dominated by herbaceous vegetation and remains waterlogged at all times. Such areas may be either freshwater or saline, tidal or non-tidal.


    an African word meaning 'rock dweller' which refers to certain species of cichlids that live in the rocky habitat of Lake Malawi.

    Mechanical filtration

    the physical removal of larger suspended/particulate waste matter from the aquarium water using a medium such as filter sponges to trap detritus, dead plant matter etc.

    Median fins

    the unpaired (anal, caudal, dorsal) fins.


    having very dark colouration because of a higher than normal level of the pigment melanin.

    Metal halide

    a type of aquarium lighting which produces powerful illumination via an electric arc passing through a bulb containing a gaseous mixture of mercury vapour and metal halides (compounds of metals with bromine or iodine). This high intensity light penetrates deep into aquaria giving a similar shimmer effect to LEDs, is long lasting, and has low running costs. However, the bulbs and their units can be a little expensive compared to other lighting types, and as this kind of lighting generates a lot of heat, a chiller may be required to keep the water temperature at an acceptable level.

    Methylene blue

    also known as methylthioninium chloride, this is a medication that is most commonly used as an anti-fungal agent when raising fish eggs.


    a medication used to treat protozoan and anaerobic bacterial fish diseases.


    tiny non-parasitic nematodes which are an excellent food for fry and tiny fish, and which can easily be cultured in wet oatmeal.


    the semen of a male fish.


    an evolved resemblance between two organisms, the mimic gaining protection for resembling a dangerous predator or distasteful/poisonous animal.


    a genus containing only one species.

    Mosquito larvae

    chironomid (non-biting) midge larvae which grow in the sediments of stagnant pools. They are a popular high-protein aquarium fish food, which most carnivores and omnivores find irresistible, usually measuring around 1cm in length.


    a fish which protects its eggs (and in some cases its young) by carrying them in its mouth.


    organic sediment/sludge that collects on the aquarium substrate, usually consisting of fish excrement, decaying plant matter, uneaten food etc.


    symbiosis which is beneficial to both organisms involved.

    Mysis shrimp

    small, shrimp-like crustaceans that provide an excellent food source for many fish on account of their high protein and fat content.


    Nano reef

    generally refers to a miniature reef system that is typically around 30 litres. Cheaper to set up than a regular reef aquarium, but can be difficult to maintain and is rather restrictive in what type and how much livestock can be kept.

    New tank syndrome

    condition in which toxic waste products (ammonia and nitrite) exist in the newly set-up aquatic system because the filter has not yet matured/the nitrogen cycle has not yet been established. Elevated levels of ammonia and nitrite will harm the fish.


    the end product of the Nitrogen Cycle. Although far less toxic than ammonia or nitrite, a high nitrate level can kill fish, and it is also responsible for excessive algae growth. Long-term exposure to high concentrations of nitrate stresses fish, making them more susceptible to disease, inhibiting the growth of young, and decreasing the likelihood of reproduction. Nitrate may be reduced by carrying out frequent partial water changes, and in planted aquaria it is taken up by plants as a nutrient.


    this is the intermediate nitrogenous compound produced in the Nitrogen Cycle. Denitrifying bacteria in the filter convert fish waste (in the form of ammonia) into nitrite, and the nitrite is then converted by a second type of bacteria into nitrate. Like ammonia, nitrite is highly toxic to fish.

    Nitrogen Cycle

    as part of their biological processes, fish secrete waste in the form of ammonia in the urine. In the wild, this is simply diluted by their vast watery surroundings, however, in the confines of the home aquarium, ammonia builds up very quickly to levels that will cause harm to the fish. Exposure to low levels of ammonia will cause the fish to become more susceptible to bacterial infections, but in higher amounts, it will burn, cause severe breathing difficulties, and kill. New filters do not contain the beneficial bacteria necessary to break down the ammonia produced by fish or from uneaten/rotting food. To begin with, although the filter can remove larger debris mechanically, it is not ready to deal with the excreted ammonia through the medium of chemical filtration. A process known as the Nitrogen Cycle sees ammonia broken down via two specific types of bacteria that will gradually colonise the filter media over the course of several weeks. This process is how the filter matures (or cycles) and subsequently helps to keep the water free of pollution. When fish are introduced to a new aquarium, there will be a spike in ammonia as waste is excreted. Nitrosomonas bacteria begin colonising the filter media, and using oxygen, convert the ammonia into slightly less harmful (but still very problematic for fish) nitrite. As nitrite begins to rise, ammonia will start to fall. A second species of bacteria, Nitrospira, then begin to convert the nitrite into much less harmful nitrate. When the filter is mature, there will be no ammonia and nitrite present, and the resultant nitrate can be diluted via partial water changes. Nitrate is not quite so deadly to fish as nitrite and ammonia, but for best long-term health, the fish should not be exposed to high levels.


    active at night.

    Nuchal hump

    an enlarged hump of tissue that develops on the foreheads of some fish, especially cichlids. These swellings can be fairly subtle, or extremely prominent, and they mostly develop on male fish. However, in some species females can also develop them, although they are usually less prominent. There is much speculation as to exactly what purpose they serve, but is thought to be an identification marker or a measure of sexual prowess.

    Nuclear family

    both the male and female parent fish cohabiting in the same aquarium as their offspring (from one or more broods).


    Obligate feeder

    a fish that has a specialised diet and can only survive long-term if a particular food is regularly provided. It may accept other foods, but won't be able to extract the precise nutrients it needs from them.

    Ocellus (plural ocelli)

    a rounded eye-like marking. Also known as an eye-spot or false eye, and is thought to be present in order to confuse predators.


    dermal teeth present on the body of various groups of fish (particularly common in catfish) and which sometimes thicken to form a dense covering of short spines, giving a 'hairy' appearance.

    Old tank syndrome

    condition in which excess nitrates, organic acids, and other products accumulate unseen in an older aquarium, causing a gradual decline in water quality and possibly a sudden pH crash. The established livestock may build up a certain amount of resistance to the changes, having been living in the tank for some time. Aquarists most commonly end up encountering this problem when new fish (that have been kept in good conditions) are added to the older tank with its deteriorating water quality. The new fish may quickly end up ill or dying and subsequently wipe out the established fish. It is then mistakenly assumed that there was a disease problem with the new fish, when in fact they were perfectly healthy but suddenly exposed to poor water conditions that they couldn't cope with. Old tank syndrome can be avoided by regular maintenance, particularly frequent partial water changes, and by testing the aquarium water at sensible intervals.


    eating both animals and plants.

    Operculum (plural opercula)

    a series of bones that provide facial support and a protective covering for the gills.


    a taxonomic category that sits below Class and above Family.


    maintenance of a proper balance of salts and water within an organism.


    a tubular organ through which a female fish deposits eggs.


    produces eggs that hatch and develop outside the body.


    eggs are retained inside the female until hatching, although they take no nourishment from the mother.

    Oxbow lake

    u-shaped section of river that has been isolated from the meandering main river channel as it found a different, shorter course.


    the process of introducing air into water, thus increasing the oxygen saturation.



    a vivarium that incorporates areas of both dry land and water, along with plants that are both terrestrial and aquatic. Ideal habitat for mudskippers and many species of crab.


    a highly toxic substance present in certain marine species including zoanthid corals. The greatest risk of palytoxin poisoning comes from exposing the slime coating produced by zoantharians to air. Wherever possible, marine animals should be handled underwater and fully submerged; they should not be lifted out of the aquarium unnecessarily.


    where an organism lives in or on another organism, to its own benefit and to the host's detriment.

    Pectoral fin

    one of the pair of fins that are each situated just behind the operculum, and normally used for balancing, braking, and propelling.


    relating to the open sea.

    Pelvic fin

    one of the pair of fins that are typically (but not always) located ventrally below and behind the pectoral fins. Used for assisting the fish in moving up and down through the water, turning sharply, and stopping quickly. Also known as ventral fins.


    a measurement of the acidity or alkalinity of the water. It is measured on a scale of 0 to 14, with readings below 7 (neutral) considered acidic, and readings above 7 referred to as alkaline or basic. The scale is logarithmic, so a change from 7 to 6 is a decrease of 10, a change from 7 to 5 is a decrease of 100 and so on. The pH value of the water is important, as different fish species prefer different conditions.


    feeds on other fish.


    feeds on plankton.


    small organisms drifting or floating in the sea or fresh water, consisting chiefly of diatoms, protozoans, microscopic crustaceans, and the eggs and larval stages of larger animals.


    a method of filtration whereby a 2-3cm water-filled void (plenum) is created between the aquarium base and a thick (10-12cm) layer of substrate, using undergravel filter plates or eggcrate covered in fine mesh/screening material. Organic waste diffuses down into the plenum, and denitrifying bacteria living in the oxygen-depleted lower level of deep substrate coverts nitrate into harmless nitrogen gas. Also referred to as a Jaubert system.


    cold-blooded, or more accurately, an organism whose body temperature varies, following that of its surrounding environment.


    a female mating with more than one male.


    both sexes mating with more than one individual.


    a male mating with more than one female.


    thriving in a wide range of salinities.


    a solitary or colonial sessile coelenterate (e.g. anemone or coral), typically having a sac-like body with the mouth uppermost and surrounded by a ring of tentacles armed with nematocysts.


    protrusion of the eyeballs. Also known as exophthalmia.


    behind (also used for 'towards the back end').


    an immature fish after complete absorption of the yolk sac, but before it has attained the appearance of a miniature adult.


    fish that migrate within fresh water only.


    a small submersible water pump driven by an impellor, and used to create aeration and current within an aquarium. A single powerhead could be positioned at one end of an aquarium to simulate a unidirectional river current, or multiple powerheads can be positioned throughout the aquarium to create a more turbulent flow.


    parts per million.


    describing an appendage that is able to grasp or hold, for example, the tail of a seahorse.


    a bony projection or outgrowth on the body.


    the stage of development between hatching and absorbing of the yolk sac.


    sequential hermaphroditism where the fish begins life as a male and then changes into a female.

    Protein skimmer

    a chemical filtration system for marine aquaria which removes dissolved organic compounds from the water before they are broken down by bacterial action. A stream of ultra fine bubbles are sent through a column of aquarium water, forming a foam that the particles stick to and which is then carried to the top of the unit where it is deposited in a removable collection cup. Also known as a foam fractionator.


    sequential hermaphroditism where the fish begins life as a female and then changes into a male.


    Quarantine tank

    a separate aquarium in which newly purchased fish are monitored for a period of time to ensure that they are free from disease before they are released into the main display tank.


    Reef aquarium

    a saltwater tank that features live corals and other invertebrates, and may or may not include reef-dwelling fish. Such a set-up replicates a section of coral reef in the home aquarium, and places strong demands on appropriate lighting, water movement, and water quality. Careful consideration must be given to which reef animals are appropriate and compatible with each other.


    a hand-held instrument which quickly and accurately measures the salinity and Specific Gravity of water by refractive index (how much it distorts light).A pipette is used to place a drop of aquarium water on the prism plate and the transparent flap closed over it. The light passing through the water sample slows and bends depending on how saline it is, and the refractometer focuses this bent light on an internal scale. The aquarist can look into the eye-piece with the prism end pointing towards a light source, and read the result.


    a separate sump tank that shares water and filtration with the main aquarium, and which usually sits underneath the main tank inside the cabinet. Useful for culturing macroalgae which will absorb waste and complete with nuisance algae (this can be regularly harvested), live food production (such as copepods and plankton), and for isolating livestock (delicate species, young fry etc) from the main aquarium.

    Reproductive guild

    a group of unrelated fish which use similar strategies to raise their young e.g. brood hiders, egg scatterers, guarders or non-guarders, livebearers, mouthbrooders, nest builders etc.

    Reverse Osmosis (RO)

    water produced by passing tap water through a semi permeable membrane under high pressure. The final product is water that is over 99% pure. It is ideal for those keeping softwater tropical fish (including discus) and marine aquaria, plus anyone who has tap water with a high pH, nitrate or phosphate levels or recurring algae problems.


    living in fast-flowing water.


    relating to or situated on the banks of a river or stream.

    River basin

    the total land area drained by a river and its tributaries.



    the saltiness or a measure of salt dissolved in a body of water. Usually measured in parts per thousand (ppt). The average salinity of the oceans is 35ppt, which means that in every kilogram (1000g) of seawater, 35g is salt. The average salinity of river water is generally less than 0.5ppt.


    small, rigid plates in the mesoderm skin layer of fishes which serve to protect, colour, and support the body. Scales vary enormously in size, shape, and structure, ranging from strong plate-like armour in boxfishes to microscopic or absent in eels.


    a group of fish swimming together in a synchronised way, with uniform distance between individual fishes, and all travelling in the same direction and at the same speed.

    Scientific name

    a standardised system used to name all living creatures, which consists of two words: the genus followed by the species. Such names are given in Latin, with convention dictating that the genus is capitalised, and both words italicised. For instance, the Common Clownfish is written as Amphiprion ocellaris. This scientific name is the same worldwide, whereas the vernacular (or common name) may vary greatly from country to country.

    Selective breeding

    also known as artificial selection, this is the process of choosing individual fish to breed with, in order to develop particular characteristics in subsequent generations.


    an organism that is fixed in one place; immobile. Sessile organisms can move via external forces (such as water currents), but they are usually permanently attached to something e.g. almost all corals, sponges, tunicates etc.


    the developmental stage where a larva is morphologically and physiologically ready to leave the pelagic environment, adopting a substrate-based lifestyle. Often, but not always, associated with larva to juvenile transition.

    Sexual dimorphism

    when two sexes of the same species exhibit different characteristics beyond the differences in their sexual organs.


    see Specific Gravity.


    a group of fish that are loosely gathered together, with variable distance between individuals, and who may be moving in different directions or orientated in various positions, forming a social group. Fish derive many benefits from shoaling, including defence against predators (less likely for any one individual to be picked off in a group), enhanced foraging success, and higher chances of finding a suitable mate. Shoaling fish may end up schooling in response to a threat.


    a tube used to move liquid from one container to another. Once the liquid has been forced into the tube, typically by suction or immersion, flow continues unaided. This is a simple way of transferring water from the aquarium into a waste bucket during water changes. In tanks with a sandy substrate, the siphon tube can be held a few centimetres above the sand, and it will suck up debris as well as water.

    Slime coat

    the continuously produced mucoprotein coating which covers the scales of a fish and causes it to feel slippery to the touch. This coating is the main defence against disease-causing organisms in the surrounding environment and also serves as a barrier to protect the loss of essential fluids and electrolytes, as well as reducing friction when swimming through the water. When a fish is under stress, the slime coat often becomes thinner, leaving the fish vulnerable to disease and parasites.

    Sneaker male

    a small, non-dominant male fish which attempts to fertilise eggs by mimicking a female in order to get close to a spawning female, suddenly darting onto the nest site and releasing milt.


    water with a low level of dissolved calcium and magnesium salts. Freshwater is classed as being ‘soft’ when its General Hardness is 8dH or lower.


    species (singular)


    species (plural)


    (n) the eggs of fishes; (v) to lay/fertilise eggs in the process of reproduction.

    Spawning mop

    an artificial spawning medium for fish that deposit or scatter eggs in or over vegetation, made from strands of acrylic wool tied together in a partial pom-pom. The addition of a small piece of cork will ensure they float with trails of yarn hanging down into the water.

    Spawning trigger

    an environmental change or cue which stimulates some fish into spawning. This is often a change in water temperature (for instance at the onset of precipitation during the rainy season) but may also occur when there is an increase in food supply, change in water level, increase in current, change in salinity, or even a change in daylight length. Not all fish have spawning triggers, some species breeding on a continual basis or only during a certain stage in their life.

    Species aquarium

    a tank in which multiple specimens of only one species are kept.


    a taxonomic rank used in the biological classification of living and fossil organisms. Species sits below genus and is the basic rank of biological nomenclature. It represents the largest group of organisms with common characteristics in which any two individuals of the appropriate sexes can produce fertile offspring. They are reproductively isolated from other such groups by behaviour, ecology, morphology, or physiology.

    Specific epithet

    All organisms are given a two-part name known as the binomial. The first part of the name is the genus to which the species belongs. The second word is known as the specific epithet. For example, Amphiprion ocellaris (the Common Clownfish) is just one of twenty-nine species in the genus Amphiprion.

    Specific Gravity (SG)

    a measure of density in comparison to that of pure water at a specific temperature. It is useful in helping marine aquarists gauge how much salt is in the water. As inorganic salts are denser than water, a solution of these will see an increase in Specific Gravity. The tropical regions of the Indo-Pacific have an average SG of 1.022-1.025. The Caribbean has an average SG of 1.023-1.026, and the Red Sea has a much higher average SG at 1.028- 1.035.


    rich in species, such as when a many species are members of a single genus.


    a mass of sperm surrounded by a capsule, which is transferred into the reproductive tract of the female. Fertilisation can occur long after mating.


    a type of nutritious blue-green algae that may be fed to both marine and freshwater fish.

    Standard length (SL)

    the measurement from the most anterior tip of the body to the midlateral posterior edge of the hypural plate (in fish with a hypural plate) or to the posterior end of the vertebral column (in fish lacking hypural plates). To put it simply, this measurement excludes the length of the caudal fin.


    an individual similar to the adult in appearance, but not yet capable of breeding.


    the material covering the bottom of a body of water or aquarium. For example: sand, gravel, pebbles, cobbles, planting mediums etc.


    living in water temperatures of 10-20°C (50-68oF), which is room temperature in most parts of the UK.


    an accessory aquarium which sits below or behind the main tank, mainly used in marine fishkeeping. A sump may be used to house the filtration, protein skimmer, heater/s, calcium reactor etc, keeping the display tank clutter-free. It also increases the overall volume of water in a system, keeping the water chemistry more stable. See also: refugium.


    where a female livebearer simultaneously carries several broods at various stages of development at the same time, enabled by the storage of sperm in the ovary. Also spelled superfetation.

    Swim bladder

    a thin, membranous, gas-filled internal organ that contributes to the ability of a fish to control its buoyancy, enabling it to maintain its current water depth without having to expend excess energy in swimming. Also known as the gas bladder.


    in decapods, these are primarily used as swimming legs, and are also used for brooding eggs, catching food, and some even bear gills. Also known as pleopods.

    Swim bladder disease

    a condition in which the swim bladder fails to function normally. Most commonly seen in fancy goldfish. The main symptom is the inability to maintain a normal, upright position in the water, which means the fish may not be able to feed properly or reach the surface. It is thought to be caused by compression of the swim bladder, due either to injury, or instigated by a distended stomach - from rapidly eating, overeating, or taking in air when feeding on floating foods at the surface. Other suspected causes include constipation, bacterial infection, and internal parasites. Swim bladder medications are available to treat any suspected bacterial issues, and we would suggest a short period of fasting (2-3 days), followed by feeding pieces of cooked, shelled peas, and avoiding floating foods.


    a close, long-term biological relationship between two different organisms, be it commensalistic, mutualistic, or parasitic.


    organisms occurring within the same or overlapping geographical areas, thus frequently encountering one another.


    an invalid scientific name which has been displaced by another now regarded as correct.


    Tail fin

    caudal fin


    fish which have been bred and raised in a home or commercial aquarium.


    Total Dissolved Solids. This refers to the amount of dissolved minerals, metals, ions, salts, and other chemicals in water, and is expressed in mg/L or ppm. Pure water has virtually no conductivity of electricity. When the TDS of the water increases, so does the conductivity of the water, and this can be measured with an electronic TDS meter. The lower the reading, the purer the water, and the higher the reading, the more solids there are dissolved in the water.


    see Subtropical

    Terminal phase

    a territory-holding, dominant male fish with a distinct colour pattern. Individuals mature initially as males or females. Under appropriate social conditions, a male or female initial phase (IP) fish may transform and become a terminal phase (TP) male. In a harem, they will only be one terminal phase male, which is essentially the alpha male or 'supermale'.

    Total length (TL)

    the measurement from the most anterior tip of the body to the most posterior point.

    Trace elements

    chemical elements which are required, usually in very small quantities, for the healthy growth of fish, plants, and invertebrates. Such elements are normally supplied through regular water changes or in marine salt mixes for saltwater aquariums, but sometimes it may be necessary to use supplements.


    a stream or river that flows into a larger river or lake.


    a disease caused by a microscopic ciliated protozoan (Trichodina sp.). These parasites, which do not feed on the fish (instead only using it as a host and for transportation), are often present in low numbers, and usually, healthy fish can control their numbers. However, in poor water conditions or when fish are overcrowded, stress causes their numbers to quickly multiply, overwhelming the fish's immune system. This can lead to secondary bacterial infections or ulcers. The first signs of an outbreak of trichodiniasis is heavy mucus production, the fish taking on a greyish white coating. This will be coupled with lethargy, scratching/flashing, clamped fins, ulcers, and in bad cases, respiratory distress. Medications are available (such as those typically used for whitespot) but improvement in water quality, a healthy diet, and a reduction in overcrowding are essential for recovery.

    Trickle filter

    also known as a wet/dry filter, this is a very efficient denitrifying filter system. Aquarium water that has first passed through a mechanical medium (to remove larger debris) is slowly trickled from a drip plate or spray bar onto biological filter media which is not sitting in water, but, as it is exposed to air, is merely damp. This 'dry' chamber increases the supply of oxygen which allows the beneficial bacteria to really thrive, thus increasing the efficiency of the biological filtration. The biological media can be ceramic or plastic (spiked bio balls, hex-nodes etc), as long as it has a large surface area, doesn't compress, and does not easily clog.


    climate that is characterised by high temperature, humidity and rainfall, and which occurs in a belt on both sides of the equator. Water temperatures are typically above 20°C (68oF).


    temporary epidermal projections or protuberances on the head, body, or fins of males of some species which facilitate contact with females during spawning or which are used for defence of territories; common amongst cyprinids.


    small annelid worms which are sometimes used as aquarium fish food. Since they are commonly found in polluted areas including sewer lines, as well as the muddy sediment of lakes and rivers, they have a reputation for introducing disease into the aquarium. It is safer to feed gamma-irradiated frozen Tubifex, rather than the live worms.


    water that is cloudy/opaque due to an abundance of suspended particulate matter.


    the rate at which the total volume of the aquarium water is passed through the filtration system. For the average freshwater tank, a rate of 4-5 times per hour is recommended, whereas much higher rates will be necessary for specialist river style aquaria, and, in particular, reef tanks. The power of a filter or pump is usually expressed by how many litres of water it is able to process per hour (L/h). For example, a filter that has a turnover of 500 L/h will push 500 litres of water through it every hour. Therefore, if you have a 100 litre aquarium, a filter with a power rating of 500 L/h will filter the entire contents of the aquarium five times every hour.


    UV clarifier

    a device which helps to clarify pond water by forcing it past a high output ultraviolet light that is protected by a clear quartz sleeve. Removes bacteria and the single-celled floating algae which causes green water.

    UV steriliser

    a device which sterilises aquarium water by forcing it slowly past a high output ultraviolet light that is protected by a clear quartz sleeve. The longer exposure time (compared to a UV clarifier) means that parasitic protozoa are eradicated.

    Undergravel filter

    a perforated tray is placed under a 2-3" layer of gravel on the bottom of the aquarium, creating a false floor which allows the gravel to be suspended above a water filled void. One or more vertical uplift pipes are attached to the tray, through which the aquarium water is drawn using a powerhead or airpump. The thick layer of substrate acts as both a mechanical and biological filter as the oxygen-rich water is drawn through it, into the water-filled void and up the uplift tubes which return the water near to the surface. The gravel has a large surface area and is ideal for supporting vast colonies of beneficial bacteria which break down the fish waste. This type of filtration is no longer very popular due to advances in technology and the availability of efficient internal and external power filters. Undergravel filters are more labour intensive to maintain, and cannot be used in planted aquaria or with species which like to dig.


    refers to certain species of cichlids that are free-roaming/open water swimmers in Lake Malawi.


    Velvet disease

    an infectious disease caused by protozoan dinoflagellates, specifically Oodinium in freshwater fish and Amyloodinium in marine fish, and is commonly caused by exposure to poor water conditions (elevated ammonia/nitrite), transportation stress, or contamination. Within hours of being infected, the respiratory rate of the fish rises dramatically, and if not treated at this stage, tiny greyish or rusty cysts appear all over the body and fins in vast numbers, giving the fish a velvety appearance. These cysts are not always obvious unless the fish is viewed head-on with light source behind it. It can sometimes be mistaken for whitespot disease, but the spots caused by velvet disease are much, much smaller (also, whitespot disease does not present with respiratory distress until several untreated days have passed). Other symptoms of velvet include the fish trying to scratch themselves against the decor, loss of appetite/weight loss, lethargy, and clamped fins. The parasite finds a fish and adheres to it using its flagellum (a slender hair-like structure which enables protozoa to swim). It then penetrates the skin and soft gill tissue, destroying the cells and feeding on nutrients inside. Once the parasite has matured, it drops off the fish and divides into dozens more parasites which then seek hosts, and the cycle repeats. This disease is highly contagious and often fatal if not spotted and treated early enough. If your fish are infected, we would suggest a large partial water change and prompt treatment with an anti-velvet medication. Also known as gold dust disease or Oodinium/Oodiniasis.


    the posterior external opening of the intestine, gonads and kidney ducts from which wastes are voided, situated just anterior of the anal fin.


    relating to the underside of the fish.

    Ventral fin

    one of the pair of fins that are typically (but not always) located ventrally below and behind the pectoral fins. Used for assisting the fish in moving up and down through the water, turning sharply, and stopping quickly. Also known as pelvic fins.


    a device designed to oxygenate a flow of water. Some internal filters have a venturi valve already built in, allowing the filter to return water to the aquarium mixed in a stream of air bubbles, keeping the water well oxygenated.

    Vernacular name

    the common name of a fish. This may differ from country to country, unlike the Scientific name which is universal.


    fish which give birth to live young, the eggs developing inside the mother whilst receiving direct nourishment from her.


    Water change

    to remove some of the water from an aquarium, replacing it with fresh water, thus diluting pollutants such as nitrate.

    Weberian apparatus

    series of 4 or 5 modified vertebrae which connect the swim bladder to the auditory system (inner ear) and convey pressure changes and sound. The structure acts as an amplifier of sound waves that would otherwise be only just perceivable by the inner ear structure alone. Found only in Ostariphysian fishes, including Characiformes, Cypriniformes, and Siluriformes.

    Wet/dry filter

    see Trickle filter.

    Whitespot disease

    also known as Ich, this is an infectious disease of freshwater fish caused by the protozoan Ichthyophthirius multifiliis. It is thought that all fish carry the whitespot parasite at low levels, but when they are under stress, it overwhelms the immune system. The fish may be stressed due to poor water quality, low water temperature, bullying/fighting, overcrowding, rough handling/transportation etc. The most obvious symptom is the appearance of small, white, clearly defined spots on the body and fins. If left untreated, these will rapidly spread to cover the whole fish. Other symptoms of whitespot include the fish trying to scratch themselves against the decor, loss of appetite/weight loss, lethargy, rapid breathing, and clamped fins. The whitespot protozoan goes through several stages in its life cycle, starting with the trophont phase where it burrows into the fish's skin or gill tissue to feed, clearly visible as a white spot (approximately the size of a grain of salt). At this point, the parasite is resistant to medications. After feeding, the trophont falls off and drops to the bottom of the tank, entering an encapsulated dividing stage (tomont). A small tiny open wound will be left on the fish and this brings with it the risk of secondary infection. The tomont adheres to the substrate, plants, wood, nets, or other ornamental decor in the aquarium. Here it undergoes a series of divisions, producing many hundreds of infectious theronts. Once the theronts are released into the water, they seek out a fish host to attack. They will then burrow into the skin and the cycle begins all over again. This life cycle is highly dependent on water temperature, and at 25°C (77°F), the entire cycle takes approximately 7 days. The infectious theronts can survive for around 48 hours if they do not find a host, and it is this phase which is vulnerable to medication. This disease is highly contagious and often fatal if not spotted and treated early enough. If your fish are infected, we would suggest a large partial water change and prompt treatment with an anti-whitespot medication. To ensure that you catch every parasite during its vulnerable stage you need to treat for the entire duration of the life cycle. Due to the nature of whitespot, by the time you have identified the disease, all the fish in the tank will have been exposed, and so they will all need to be treated. Slowly raising the temperature of the water to around 30oC (add extra aeration) can be very helpful as it speeds up the life cycle, but not all fish can tolerate higher temperatures, so do research carefully. Also, be aware that many effective whitespot treatments will harm scaleless fish, so tanks containing such species may require half dosing. Please speak to a friendly member of staff if you are unsure about anything. Prevention is the key when it comes to whitespot: ensure good water quality at all times, quarantine any new fish, and eliminate any possible stressors. Installing a UV steriliser on the aquarium will also help to avoid further outbreaks. Equipment such as water change hoses, nets, buckets etc that are used at the time of infection will need to be sterilised with boiling water.


    refers to either the frothy, highly oxygenated water in rapids, waterfalls etc, or silt-rich water which is almost opaque.



    having yellow pigmentation.


    feeding on wood.


    Yolk sac

    in larval fish which are not yet able to feed themselves, this is a bag-like, ventral extension of the gut containing nourishment. Once the yolk sac has been fully absorbed and the young are able to feed themselves, they are termed free-swimming fry.



    is a naturally occurring, highly-absorbent, porous mineral that binds ammonia and heavy metals in freshwater through an ion exchange mechanism. This can be placed in a bag in the filter, but must be regularly replaced or recharged by soaking in a strong salt solution for 24 hours. Useful for temporary ammonia problems, hospital tanks (where medication can harm the biological filtration), and when transporting fish on long journeys. Not recommended for long-term use.


    an early larval form of crabs and many related decapod crustaceans, characterised by the use of thoracic appendages for swimming and a large dorsal spine.


    a symbiotic dinoflagellate present in large numbers in the tissues of many marine invertebrates such as anemones, clams, corals, jellyfish, sponges etc. These microscopic algae are photosynthetic organisms which utilise sunlight to provide essential nutrients to the hosts. In return, the host provides a protected environment for the algae to live as well as compounds they need for photosynthesis.

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