An aquarium heater isn't just for raising up a fish tank's temperature to a warmer level. It is also used to maintain a stable temperature in the aquarium. Keeping a stable temperature with a submersible or hang on aquarium heater is absolutely critical. Changes in aquarium temperature can have a big effect on the feeding habits and color of aquarium fish. Heaters have become easier to use over the years, including built in thermostats and temperature controls which can be dialed in and then left alone to keep the water at a set temperature.