A good quality aquarium heater is a must in saltwater fish tanks, especially for your fish. Temperature fluctuations can cause a lot of problems with fish and some corals. Fish can become easily stressed if the temp is changing. A good rule of thumb is to keep your tanks temp within 2 degrees or less. For example, if your tank is running 78 degrees with your lights on, and they go off, make sure your heater keeps your tank at least 76-78 degrees. Also, when shopping for a heater, it is customary to get 3 to 5 watts for every gallon.