The new, easy-to-use Tetra HT10 and HT30 Submersible Heaters are created especially for aquariums ranging from 2 to 30 gallons. The Tetra Submersible Heater contains an electronic thermostat that automatically adjusts the heat output to keep the water at a constant 78°F. All you have to do is put it in the aquarium and plug it in!