Tetra'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 thermostat maintains water at 78°f without any adjustment required. Be sure purchase the correct heater for your tank size. Buying a heater too small for your tank can cause under heating. It is critical to ensure your tank is equipped with a hood or glass canopy to contain the heat inside the aquarium. All tetra ht heaters will shut off if an electrical short is detected for your safety.