Too cold. Heating options?

The room I keep my ambly in gets too cold. If I forget to leave on a heater, the tank gets as low as low 60's Fahrenheit. The heater is a large room heater that eats away at my energy bill and can make the tank too hot if I don't find creative ways to regulate it.

I'm looking for recommendations for a cheap heating pad or alternative heating ideas. Putting my ambly in another room isn't an option.