Tetra's 100 Watt Submersible Heater is designed for 10 to 30 gallon Aquariums. UL listed. Electronic thermostat automatically maintains water temperature. Prevents overheating, extending heater life. No adjustments necessary. The indicator light lets you know when the water is being heated (red light) and when it is at the proper temperature (green light). Silicon Carbide crystals improve heat distribution and transfer. Strong glass tube is heat and shock resistant.