Interpet Deltatherm Aquarium Fish Tank Heater, adjustable, for Tropical and Marine, 25 W, for aquariums up to 30 litres,BlackPrecise and reliable heating control for tropical aquariums, freshwater and marine Keep your aquarium at exactly the right temperature with this high performing heater Extra safe, double insulated and fully submersible, made with heat and shock resistant glass The positive click control makes it easy to adjust the temperature precisely, but as the heater is already set to 24C you might not find you need to 25 W model for aquariums from 4 to 30 litres,