Aqueon Submersible Aquarium Heaters are made from thermal shatter-resistant quartz glass and designed to absorb impacts, making them extremely durable. An electronic thermostat stops heating once set temperature is reached. A power indicator light turns on when the unit is heating and is equipped with a 6-foot power cord. UL approved, the heater is also programmed to turn itself off when not properly submersed to avoid overheating and damage. This heater can be positioned at a vertical or horizontal angle and used in freshwater and saltwater aquariums.