Aqua Innovations 300w Submersible Aquarium Heater with LED Display
If you are keeping tropical fish, you will need a heater. A heater insures that a tank doesn't get too cool, and that the temperature stays steady during the course of the day, even when the room cools off (e.g., at night). For many tropical fish, a temperature of 78F is ideal. The built-in thermometer will help you verify that your tank stays at its proper temperature.