25Watt Heater：This Aquarium heater is suitable for 1 to 8 gallons aquariums50Watt Heater：This Aquarium heater is suitable for 1 to 15 gallons aquariums100Watt Heater：This Aquarium heater is suitable for 10 to 30 gallons aquariums200Watt Heater：This Aquarium heater is suitable for 30 to 50 gallons aquariums .Automatic constant temperature：When the water temperature reaches 78°F, the green light is on, stops working and will automatically keep the water temperature of the fish tank at 78℉. This temperature is very suitable for betta and most tropical fish. When the water temperature is lower than 78°F, the heater automatically starts to heat and the red light is on. Easy to Use-you can use the two suction cups to fix the heating rod and easy to hide in the fish tank. You can switch the red temperature control knob on the top to adjust the temperature. Vertical or horizontal placement into the water using,during its heating time, heater must be 100% submerged into the water.