Cyprinus carpio
DistributionEastern Asia
Sexual DimorphismIn fish over 25cm (10”) females will be fuller bellied, whereas males will remain more streamlined. Males ready to spawn will develop breeding tubercles on the head & pectoral fins.
Maximum Size90cm (35.4”)
Water ParametersNeutral to alkaline conditions preferred. pH: 7.0-8.5, dH: up to 25 degrees.
Temperature2-30 deg C (36-86 deg F)
CompatibilityLarge fish community
LightingNo special requirements

The ornamental koi that we know and love today are descended from the Black Common Carp or ‘Magoi’ (Cyprinus carpio). Over the centuries, these majestic fish have been line bred for numerous colour mutations, leading to some quite outstanding varieties. Koi are always best kept in a pond situation, especially if they are to reach their full growth potential through exercise gained by swimming. The pond should be at LEAST 4ft/1.2m deep (6ft/1.8m or more is better) and also as long and wide as possible to give a large surface area for oxygen exchange. In colder climates, it is important to consider the depth of the pond because of seasonal temperature fluctuations; rapid changes in temperature will distress koi. The deeper your pond is in proportion to the surface area, the less susceptible it will be to swings in temperature. A larger volume of water is also, of course, much more stable in terms of water parameters than a smaller one. A pond heater is definitely worth considering for the winter months for those in colder climes. There is much that should be planned out thoroughly beforehand, such as location, size, shape (formal or non-formal design), materials (liner, concrete etc), and the need for oversized filtration (gravity fed/pump fed etc) and a power supply. A vital consideration for a healthy pond is choosing a suitable location. Try and choose an area that is of south/west aspect and which is in medium shade, receiving approximately 4-6 hours of sunshine per day. An unshaded north facing garden is far from ideal. Ensure the site is not directly under the shade of trees as falling leaves, and the invasive root system of some species can become a problem. Sufficient area must be left for a large filtration system (with a size at least 1/3 the surface area of the actual pond), and this is an area that shouldn’t be overlooked in the haste to construct the pond. A filtration system cannot be too big but it can cause a lot of problems if it is too small! Experts are always on hand at your local Maidenhead Aquatics to discuss your options when it comes to filtration for the koi pond. Other aspects to think about are whether you wish to incorporate some form of waterfall or fountain, which will aid aeration during warmer days. Once the pond has been constructed, it should be filled with dechlorinated tap water (rain water is unsuitable for the first fill as it lacks the necessary minerals). Marginal plants such as irises can be grown at the sides of the pond if provision is made during the building for some shallow shelves around the edge. Koi tend to uproot much plant life in deeper areas through their natural browsing of the bottom of the pond in their search for food items. Rocks would need to be placed towards the front of the shelves to prevent the koi from getting at the marginal plants and harming them. The rocks will also hold back the planting medium and stop it from entering the pond, whilst still allowing the passage of a small amount of water to keep the plant roots nice and moist. These type of plants can be a valuable aid in helping to remove nitrates from the water (although there is no substitute for powerful filtration). There are koi-resistant planting containers that allow plants such as lilies to be grown in the deeper areas of the pond. The wide leaves will provide welcome shade for the fish on really sunny days. Try to change approximately 10% of the water per month (rather than just topping up) with dechlorinated tap water, as this will help to reduce nitrate and replace essential minerals. Water should be tested on a weekly basis as this will alert you to the first sign of any problems. UV clarifiers can help if green water becomes a problem; although koi do actually enjoy this type of environment (browsing for the tiny animals that eat the algae), it can look a little unsightly and make viewing the fish difficult. Not only will this spoil your enjoyment, but any koi that might have injured itself or is suffering from parasites, for example, will remain hidden from your attention. Ensure that you do not overstock your pond, making sure you allow for the eventual size of your koi, and always quarantine new purchases.


Good quality pond flakes (for smaller koi), pond pellets, pond sticks, frozen/live foods etc. Prawns, earthworms, and lettuce are all good treats to be fed now and again. As the water temperature cools down in late Autumn, a lower protein wheatgerm-based food should be fed.


Spawnings will occur naturally in the pond if sexually mature males and females are present, where conditions are to their liking (water quality, water temperature, light levels etc), and where there is a medium such as mops/ropes/brushes where the eggs can be scattered. However, many spawnings go unnoticed as the eggs are often consumed quickly by the parents and other fish in the pond (and even other predators such as dragonfly larvae, snails, and tadpoles). To ensure survival of the eggs/fry, the aquarist will usually have to intervene, collecting the eggs before they are devoured, and keeping them in a spacious container with pond plants and some continuous form of gentle oxygenation. The eggs typically hatch within 4-5 days, and the fry will feed on their yolk sacs for the first few days, after which time a constant supply of small live foods such as infusoria and daphnia will be required. Koi, of course, can be selectively bred in separate breeding ponds if the aquarist is fortunate to have access to this type of set-up. In this scenario, the water of the breeding pond should match that of the main pond, and it should be lined with spawning ropes and the like. A well-conditioned selected pair can then be added and observed regularly for signs of spawning. Once the eggs have been deposited over the spawning medium, these can be separated from the parents and grown on in an independent container as described above.

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