Tetra s HT line of heaters are easy-to-use and work in aquariums ranging from 2 to 55 gallons. The HT10 and HT30 have electronic thermostats that automatically adjust heat output to keep water at a constant 78 degrees F. The HT55 is adjustable between 72 degrees F and 85 degrees F. All HT heaters have indicator lights to let you know when the heater is on heating the aquarium.