If you are purchasing tropical fish for your tank, you will need a heater to control the temperature of the water, this is also important for most freshwater fish. Some heaters do not come with a thermostat, so it is important to purchase a thermomoter if your heater only has a low to high control.
As a general guide most fish tanks require the water temperature to be between 22-26 degrees Celsius. It is always best to check the ideal temperature when you are purchasing fish for your tank. When choosing the correct heater wattage it is best to select one based on the actual water volume in the aquarium, this is generally 1 watt per litre of water. This is why it might be better to overestimate your heater and purchase one that will automatically shut-off at the ideal temperature.