The Science of Aquarium Heaters
A heater is necessary to stabilize the temperature of your aquarium water. Even if you live in a warm climate, the temperature changes from day to night are enough to stress your fishs' immune system and leave them vulnerable to diseases.
A minimum of 4-5 watts of heat per gallon of water is recommended. If you have a large aquarium, two heaters are better so temperatures throughout the tank can stay stable. Also, two heaters give you redundant heat in case of aquarium heater failure.