I will probably have to get a small one for my 2 gal pico tank. We heat with wood, and we usually keep the bedrooms cool during winter, so I have been thinking of one of those pad type heaters by Hydor...........which is about 7 1/2 or so watts and supposed to heat water to a max of 5 degrees above room temp....no adjustable thermostat control.......just the built in differential control, so combined with the PC lights heat and the heat pad I think it will put me in the ball park temp wise. I think it will fit perfectly inside my AC20 hob filter/fuge