It is really important to keep a fish tank at the ideal temperature for its inhabitants. Too high or low, and your fish will really suffer health-wise.
Fish are cold-blooded animals, so they cannot control their own body temperature. They need the environment to stay warm and cold. If the water is too warm, the metabolism and breathing of fish speed up, so they use more oxygen. But the warmer the water, the less oxygen it holds, so they can struggle to breathe. This causes stress.
But the best temperature for a fish tank depends on the type of fish, and where they are native to in the world.
It is important to note that every type of fish has a temperature scale as a guide. This can be good if you have a range of different species in your tank. However, if you have speciality fish you need to try and research the breed and the exact temperature they need.
What temperature should a tropical fish tank be?
Tropical fish do best above 20°C, which is around 68°F. Most maximum temperatures will be around 30°C. For those at the higher end, an aquarium heater will most definitely be needed for regulation and control of the water.
Tropical fish can then be split into three separate sub-groups. Some will need temperatures of the lower 20°Cs, some mid-20°Cs and others high 20°Cs.
For example, Discus need the high 20°Cs, around 28°C. But tropical fish which require the lower 20°Cs, such as white mountain minnow, can technically be classed as temperate now (more below). You may still see them referred to as tropical in some pet shops, though.
Freshwater and saltwater fish
Tropical fish will often be split into two groups: freshwater or saltwater.
Freshwater fish are adapted to cope with a wider range of temperatures, in both tropical and cold water climates. They are often found in lakes, rivers and wetlands, where salinity is less than 1%.
Saltwater fish (also known as marine fish) can live in tropical climes or the cold Antarctic and Arctic oceans, but fish we keep as pets are commonly from warmer sea climates. Therefore, if your chosen fish are saltwater, they may need slightly higher temperatures than a freshwater fish would.
What temperature does cold-water fish need?
Cold-water fish species prefer temperatures below 70°F, which is below 21°C. This is why some fish classed as tropical fish are actually better in temperate, non-heated water.
Usually, a fish tank for cold water fish will not need a heater. Most homes in the UK are definitely warm/cold enough for this, and as long as the tank isn’t near any heat sources or windows, it should remain consistent throughout the year.
What temperature should a temperate tank be?
Temperate fish is a relatively new fish type in the world of home aquariums. As homes in the UK have become warmer with central heating and insulation, tropical fish requiring temperatures in the lower 20°Cs will likely not require heaters.
Temperate fish can be found in habitats that overlap tropical and cold-water needs. They’ve evolved to live in temperatures of 14–24°C.
If this sounds confusing because of the crossover, you’re not alone. But in simple terms, they can cope with temperatures a little bit warmer than the maximum temperature for cold-water fish. However, they can’t cope with temperatures as high as most tropical fish require.
A lot of people in the UK opt for temperate fish because they don’t need heaters, yet don’t need to worry about the tank becoming warmer with central heating or the summer weather.
Do my fish need a heater?
If you’re keeping tropical fish, you will definitely need a heater. Any other kinds, and there’s a chance you will be able to get away without having one in the UK. The water in a fish tank can often cool or warm to match the temperature of the room.
However, always check your particular fish breed’s needs. Also, think about your home and where the fish tank will be.
Fish tank heaters are often preset to 25°C. This is what the average tropical aquarium will be. Some people won’t move their heater from this preset temperature, which is fine for most fish. But if you do need things to be more precise, you will need an adjustable heater. These can often be set between 20-30°C (68-86°F).