Tetra HT Submersible Aquarium Heater: HT10 Heater - 50 WattTetra's HT line of heaters are easy to use and work in aquariums ranging from 2 to 55 gallons. The heaters have electronic thermostats that automatically adjust heat output to keep water at a constant 78 degrees F. All HT heaters have indicator lights to let you know when the heater is on, heating the aquarium. All Tetra HT heaters have red green indicator lights to let you know when the heater is heating(red) or on standby(green). The low voltage