Tetra HT Submersible Aquarium Heater With Electronic ThermostatAll 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 78F 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