Aquarium tank heaters come in different sizes and are measured in watts. To determine the size of heater for your aquarium multiply the number of gallons of water in your aquarium by 5. For example a ten gallon aquarium needs a 10×5 =50 watts heater. Never remove a hot heater from your aquarium. It may over heat and crack or crack when it goes into the water. Always unplug the heater 15 minutes before removing it and be sure to plug it back in after you put it in the aquarium. An Aquarium Tank Heater assists in maintaining the ideal environment of your aquarium and is available as hang on or submersible heater.