All Tetra HT heaters have indicator lights to let you know when the heater is on. It will be red when heating and green when the proper temperature has been reached. The HT10 uses a built in electronic theromstat to automatically maintain water at 78° F which is ideal for most tropical fish. No adjustment is requried. Ideal for aquariums between 2 to 10 gallons with hoods or glass canopies The HT heater is fully submersbile and can be installed vertically or horizontally. It's small footprint makes it easy to conceal behind plants or other décor. Please read all label information upon delivery. DO NOT plug heater into an electrical outlet until it is placed inside the aquarium.