25Watt Heater：This Aquarium heater is suitable for 1 to 8 gallons aquariums 50Watt Heater：This Aquarium heater is suitable for 1 to 15 gallons aquariums 100Watt Heater：This Aquarium heater is suitable for 10 to 30 gallons aquariums 200Watt Heater：This Aquarium heater is suitable for 30 to 50 gallons aquariums.