Experts recommend five watts of heat for every gallon of water, but this depends on whether you need to heat the water up to 10 degrees above the room temperature and that you are using a tank lid to retain the heat and prevent cooling by evaporation. If you live in a particularly cold location, you could need more than five watts per gallon to get the tank water up to the required temperature for your tropical fish collection. If in any doubt, chat with your aquarium pet store for advice.