Tetra's HT line of heaters are easy-to-use and work in aquariums ranging from 2 to 55 gallons. The HT40 has an electronic thermostat that automatically adjust heat output to keep water at a constant 78 degrees F in aquariums 30 to 40 Gallons. All HT heaters have indicator lights to let you know when the heater is on, heating the aquarium.