"This heater is effective for small tanks that need the water to stay a little warmer than room temperature.
The pros are: you can either attach this heater to the side of the tank with a suction cup OR you can completely hide it under your gravel, and just have the wire sneak up the corner of the tank.
The cons are: there is no way to adjust the temperature or have it turn off automatically. It is either on or off, and heats the water only a few degrees."