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.
Selecting the right heater.
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.
Types of aquarium heating
Heaters without built-in thermostat - this are a heater rods which are difficult to buy nowadays. This type of heater is not recommended as only source of the heat in the aquarium. This type of the heater can not keep stable temperature in the tank. There should be external temperature controller used with this heaters.
Heater with integrated thermostat - the most common type of heating due to the affordable price and its properties (uniform design, the ability to regulate the temperature - accurate precision and temperature stability). Of course, this heater does not provide 100% maintain a constant temperature in the aquarium but it is the best solution. Different models can be more or less accurate. Cheaper models can have bad sensors which will not keep right temperature - also they life time is shorter so it is good to invest more money in this equipment.
Heating cables - silicone cables, which are used primarily for heating the substrate. Placed in the substrate or directly on the bottom glass of the tank. This type of heating also cause the water to circulate freely in the substrate which is beneficial for the plants roots.