Having constant temperature in the aquarium does not only mean water temperature should be stable. It also mean that air above the water and substrate should have constant temperature similar to water temperature. Fish, however, are ectothermic animals (adapt to the temperature of the environment they are in), but exceeded certain limits of the temperature range impacts on their health. For fish thermophilic optimum temperature is 22-28°C. That is why we need to heat water (average room temperature is 18-21ºC). Cold-adapted fish (including domestic species) like optimum temperature not to exceed 20ºC, and in winter should not exceed 5°C. To heat water in aquarium an aquarium heater should be used.
The main parameter of the aquarium heater is it's power in Watts. The selection of the correct power of the heater is influenced primarily by the size of the tank. There are also other important factors like: presence of the tank cover (suitable cover can provide excellent thermal insulation), the type of the central heating, presence of the thermoregulator. There is a principle for small and medium-sized aquariums (up to 80 litres) that every 1 litre of water should be heated with 1 W of heater power. For large tanks the power required to heat the water is smaller. Large aquarium should have 0.5W of heater power per litre.