Hearter???

For a tank that small you could probably use a small sticky reptile heater pad, just stuck to the outside or bottom of the tank.

Something like this would probably serve your need.
http://www.thatpetplace.com/Product...rs/T1/F54BA+0418+0016/EDP/44886/Itemdy00.aspx

A small 25 watt aquarium heater will work just fine. It would take a lot to melt the tank with an internal heater just do the the fact that the water will difuse the heat and keep the plastic at a constant temp. The pads that go under would work also, but you will need more heat to do the job just do to the insulating factor of acrylic.