I have a very old 5 gal tank that has a slate bottom (very
old) and is also up on little legs so there is a space underneath. I'm using a reptile heater under the tank and I have it set up with a Zoomed thermostat : http://www.drsfostersmith.com/produc...44&pcatid=6544
It's not really a thermostat, though, it just allows you to turn the heater up or down.
It seems to be working fine. It holds my tank at 72-75 when my house gets down to 62 at night. I will probably have to turn it off in the summer, though.
I think these heaters are not recommended for glass bottom tanks though, because of the weight that will be sitting on them (unless you have the kind of stand that just supports the rim and has a gap or opening under the rest of the tank.
I also think that the slate bottom of my tank radiates the heat nice and even - don't know about glass.