The water temperature of your pond, or aquarium, plays a fundamental role in determining how active fish are. This in turns will have both a direct and an indirect affect on water quality. Fish, unlike mammals and birds, are unable to control their own body temperature. They are scientifically referred to as poikilothermic. Their metabolism reflects the temperature of the water in which they live. As a consequence, they will show a wide range of responses to all biological processes such as feeding and breeding according to the temperature of the surrounding water. But what is the best temperature to keep our fish?
Go to any pet store and you will see that their fish have been divided into two main groups, namely tropical fish and cold water fish. This division must be interpreted with a little bit of care.
Generally speaking these fish require a heater to prevent the water temperature from dropping too low. If it does it may prove lethal to the fish. A safe range could be expected to be 20 – 28°C. Every species however, has evolved according to its natural habitat. If the water temperatures naturally fluctuate then the fish have the ability to tolerate a much wider range of temperatures. Discus thrive best at the higher range of temperatures whereas species such as white cloud minnows prefer a much lower average.
Marine fish temperature tolerances
The ocean temperatures vary far less than freshwater lakes and rivers. As a consequence marine fish generally have more precise temperature requirements. Certain specific habitats however, such as rock pools, may experience much wider variations in temperatures. Fish that live in rock pools, such as gobies and bennies, naturally have a much greater temperature tolerance than fish that live on a coral reef for instance.
Fish found in the cold water section of your local pet store generally do not require additional heating. They are able to tolerant significant drops in temperature – usually down to freezing. However, they may well also be able to thrive in the higher temperatures experienced by so called tropical fish. In fact some cold water fish may actually prefer so called tropical temperatures.
A good cold water aquarium fish is one that is able to tolerate a wide temperature range. Goldfish would be a classic example. But they nevertheless feed most and grow much more rapidly if the aquarium temperature is around 22°C. Very often fancy goldfish are kept in tanks alongside the tropical fish.
Water temperature controls fish feeding levels
Low temperature result in low feeding and growth rates. Below 10 °C. a carp will rarely attempt to feed and certainly would not grow however much food is available. At 12 °C. carp will start to show some interest in food but will not grow appreciably until the temperature exceeds 15 °C. Rather more surprisingly the temperature at which carp grow fastest is a tropical 28 °C. A temperature rarely experienced in the UK for any length of time. Perhaps more importantly a carp will convert its food most efficiently at a temperature of around 25 °C. I have always maintained my carp systems at 24 – 25 °C. when maximum growth rates have been the main objective.
Optimal fish tank temperatures
Each species of fish will have specific temperatures at which feeding starts, growth is at a maximum, feed conversion is optimal, spawning takes place and death occurs. The control of water temperature is therefore a vital parameter if optimal growth is wanted through out the year. This is certainly not something you have to achieve. Seasonal variations are the norm and heating fish tanks to 25 C. in the winter can be prohibitively expensive. Carp in outdoor ponds in the UK usually take 3 years to reach 1kg. In the tropics the same size can be achieved in something approaching 10 months.
|Species||Optimum Temp. °C.||Tolerable Temp °C.|
|Trout||14 – 16||10 – 18|
|Goldfish||18 – 20||6 – 26|
|Fancy Goldfish||22 – 26||6 – 28|
|Carp, koi||24 – 26||12 – 30|
|Tilapia||28 – 30||20 – 35|
|Channel Catfish||28 – 30||20 – 35|
Optimal biofilter temperatures
The community of microorganisms living within the biofilters will also have their own preferred temperatures. Again their metabolism, and hence ability to breakdown ammonia, will be dependent, partly at least, upon water temperature. Fortunately they have similar optima as the more popular choices of warm water fish for aquaponic systems. Nitrosomas, a major ammonia oxidising bacteria(AOB), is thought to be most active at 28 °C. Activity falls off steadily as the temperature drops. In fact, below 5C. nitrification grinds to a halt. Biofiltration is so useful as a means of controlling toxic levels of ammonia largely because it’s efficiency naturally balances with the rate at which waste products are produced. The higher the temperature the more the fish feed, the more ammonia is produced and the more ammonia is broken down by the AOB.
Heating systems for fish
Heating is the only way to maintain suitable temperatures for many of our favourite aquaponics fish. Cost is normally the overriding problem. Although it is cheaper to heat a fish tank than to cool it does require careful design if energy costs are not to spiral out of control. The following points are well worth considering.
- good insulation is paramount to minimise heat loss. If you are considering installing a heating system always, always invest in the best insulation that you can afford.
- prevent wind chill. Even gentle winds can quickly ‘suck out’ the heat from a fish tank. In practice only the most affluent would consider heating an outdoor pond.
- minimise evaporation. Evaporation is a principal method of cooling. It needs to be controlled. A simple cover over the aquarium or pond is essential. They have the added benefit of preventing fish from jumping out. Something that can easily happen to our biggest and most prized fish especially when first put into a new tank.
- minimum influx of clean, unheated water. Most natural water sources will be at a lower temperature than the fish reading tanks. Water changes are usually an important part of maintaining excellent water quality but try to keep them to a minimum for the sake of your heating bills. Clever design would utilise a heat exchanger to minimise the heat loss of the water flushed out of the fish tanks.
- minimum tank volume. Not usually a major consideration but obviously a smaller water volume is easy to heat than a larger on. Tropical fish are for the aquarium, not the outdoor pond!
Good insulation is essential
With adequate insulation it is perfectly possible to maintain water temperatures around 25 °C. I would not however attempt this out doors. Some kind of building is essential. A poly tunnel or green house would be well worth the investment, even if additional heating was not installed. The systems I have built and managed have all been built in a mushroom type poly tunnel around 1,000 sq. ft. in area. This is essentially a poly tunnel with 150mm of glassfibre insulation sandwiched between two outer layers of polythene. The heat loss was so low that a temperature of 25 °C could be maintained for 9 – 10 months of the year just from the heat of a 0.75kW. water pump. Rather surprisingly heating costs, although significant, can often be maintained below those of pumping. Just don’t underestimate the importance of insulation.
General recommendations for optimisation
Temperature is so important that anything we can do to help elevate water temperatures will help promote more rapid fish growth. There have been many ingenious schemes to help achieve this including ground source heat pumps, wood pulp burners and even tapping into the heat generated from compost heaps. It is the latter that I want to investigate further as they also offer the potential to recycle the fish poo that is produced in such large quantities.