The size of aquarium heaters is measured in watts to indicate the amount of power – and heat – they can generate. Various factors will help you to determine the right size of heater for your aquarium, including how big or small the tank is and how many gallons of water it holds. You also need to factor in the average air temperature of the room your aquarium is in – the colder the room temperature, the bigger the difference between that and the required water temperature for your fish, so the larger the heater you'll need.