Overview The Aqueon Mini Heater is ideal for small aquariums, up to 5 gallons in size. When used on aquariums between 2 and 5 gallons, it keeps the water temperature at the ideal range for tropical fish, 5 to 10 degrees Fahrenheit above ambient air temperature. Fully submersible and shatter proof design keeps your fish safe. Thermal safety fuse shuts off the heater when it dries out or in any other event it might overheat. Never raises the temperature above 80 degrees Fahrenheit.