Tetra HT Submersible Heaters are easy-to-use and automatically maintain aquarium water temperature. Submersible Aquarium Heaters feature an electronic thermostat that adjusts heat output to maintain constant water temperature. Pre-set heater technology reduces the risk of overheating and prolongs the life of the heater. Indictor light allows simple visual confirmation of heater operation. Units display a red light when the water is heating a green light when water is at the proper temperature. Tetra Aquarium Heaters are made of heat and shock resistant glass and come with a 6 foot cord.