Hydor Slim 7.5 watt aquarium heater is great for micro and mini aquariums from 2-5 gallons. The flat, oval shape allows you to place this slim fish tank heater out of sight in a variety of tanks, including glass, acrylic or plastic. It helps increase the temperature by a few degrees (4ºF-5ºF) above room temperature. Fully submersible and UL listed. Your heater should be between 3 to 5 watts per gallon of water. Measures 4 1/2" x 2 1/2" x 12".