Aqueon Submersible Aquarium Heaters are made from thermal shatter-resistant quartz glass and were designed to absorb impacts, making them extremely durable. A LED indicator light comes on when unit is heating. UL approved, the heater is also programmed to turn itself off when not properly submersed to avoid overheating and damage. Electronic thermostat ranges from 68° to 88°F. The suction cups make horizontal or vertical installation quick and easy.