Aquarium heaters are an essential piece of equipment for the successful keeping of tropical and marine fish. It is important to keep the temperature of the water in your fish tank stable, failure to do so will stress fish and can lead to illness and disease.
As a general rule your aquarium or tank will need 1w of power per litre of water. This makes selecting the correct size heater for any given size of aquarium relatively simple. For example a 100 litre tank will require a 100w heater. However if your room gets particularly cold it would be a good idea to choose the next size heater up. Having said this it is also important not to put to large a heater in a small tank as doing so will increase the aquarium temperature to quickly causing undue stress to its occupants.
Failure to follow the above advice could result in a damage heater. Either putting a hot heater into water or removing a heater before allowing it to cool down can cause it to crack.