It’s that time of year when the heating goes back on, and this is something that many of us have in common with our pets. To ensure they remain healthy, it’s important to know that your aquarium heater is working properly and is set to the right temperature for the fishes you keep.

Speak to anyone who’s considering keeping an aquarium for the first time and they’ll often reveal that they’re anxious about keeping tropicals because of the heating. This is a bit like not buying milk because your house is too warm and the answer to both situations is fuss-free technology. Modern aquarium heaters are extremely simple to use and very reliable. Once they’ve been set to the desired temperature, they can be left to do their thing – much like a fridge.

Traditionally, every fish keeper killed at least one heater as a rite of passage, usually by letting it overheat out of water. Back in the old days, heaters were more primitive tech and a bimetallic strip mechanism gave crude precision that could go disastrously wrong if an algae magnet was placed too near, or if the contacts became pitted and worn. Nowadays, electronic components give greater accuracy and many units come with an automatic shut off that prevents terminal overheating. This protects them from the classic scenario where a heater is left exposed to air during a water change or unforeseen drop in water level. It’s still good practice to unplug the heater when performing a water change as this is a vital part of your life support equipment after all. No conversation about heater failure is complete without mention of the old safety procedure of using multiple small heaters, as this was a safeguard against the prospect of one larger unit failing and overheating the aquarium – a similar failure in a lower wattage heater takes far longer to warm the water to a temperature which is lethal to the inhabitants. The other benefit of proportionally small heaters is that their longer heating time means that they’re not being turned off and on as frequently by the thermostat, leading to less wear and tear of components. There’s still wisdom in this approach.

In terms of maintenance, heaters don’t ask for much but may need cleaning occasionally and sometimes descaling as a result of being exposed to hard water, biofilm or contact with the substrate. If a thorough clean is required, unplug the unit and allow it to cool before placing it in a solution of pond pump descaler or similar. It’s important to remove any mineral deposits which might lead to localised overheating of the glass tube. Of course, nothing lasts forever and it’s wise to retire your heater from active duty before it gets old enough to be unreliable. As a crucial piece of equipment with grave consequences if it fails, we’d suggest replacing your heater at least every ten years. As hardware failure tends to happen at the worst possible time, having a spare heater is always prudent for those Christmas/holiday departure/hospital visit moments when buying a replacement might not be convenient.

When selecting your heater, a couple of factors need to be born in mind. Firstly, the difference between the room temperature and the aquarium dictates how much work you’re asking the equipment to do. If you like to keep a cool home, you may have an aquarium 10c above room temperature and this would be a good reason to round-up the manufacturer’s recommendations. Conversely, having too large a heater and poor water circulation might combine to give a situation where the thermostat fails to keep the required temperature due to the time taken for the element to cool. For this reason, it’s important to place your heater in a good current, at an angle to disperse heat as efficiently as possible. Don’t be seduced by the relatively small incremental price between one that’s the right size and one that’s more powerful, remember it’s better for the life of the thermostat to avoid being turned on and off continually. Other issues which can have some bearing on heater selection are the use of cold water when topping up after a water change or running an open-topped aquarium which loses heat quickly. A heater can be relied upon to restore the set operating temperature and in some tanks, such as those used for breeding, this can be part of a standard routine. The smaller your heater, the longer the cooling period whilst it catches up.

For most aquarium residents, temperature can be divided into three bands. Temperate fishes are often subtropical and only need heating when things get cold enough to put a jumper on. Together with species from high altitude habitats, they may be happiest at the bottom end of the dial, around 18-22c. This might mean that a lower wattage heater is sufficient to give the modest increase they need and is more easily hidden. The majority of tropical fish are happiest between 22-26c with a little variation giving better results in terms of growth or spawning. The long-established recommendation of 25c is what usually influences heater selection, and this may be only a couple of degrees above some household room temperature levels on cosy winter nights. Some specialised groups of fishes originate from warmer areas, meaning that L number plecs and even domesticated discus are best kept between 26-32c with aeration used to combat the lower oxygen levels which can result from high temperatures.

What’s worth remembering is that none of these tropical temperatures seem particularly warm to us mammals – you certainly wouldn’t want a bath that cool. This is why it’s always a good idea to use a thermometer to check that your heater is accurate This is especially important if your pets are lethargic or showing signs of stress-related ailments such as whitespot.