AQUARIUM HEATER: Submersible heater is designed to keep your aquarium at the optimum temperature. AUTOMATIC THERMOSTAT: Electronic thermostat keeps water at a consistent 78 degrees. INDICATOR LIGHT: The red light shows when water is heating; green indicates that the heater is on standby. EASY TO USE: Just attach to an aquarium wall – the suction cup securely attaches heater to tank. SIZE: Use this 100-watt heater for 10 to 30 gallon aquariums – UL Listed.