RustysDragons has it pretty close. Thermostats use a mercury switch to activate them (well MOST do, i've seen some that don't, but most do)... when the mercury cools down it moves to one side because its heavier or lighter (i forget which) and vice verse when it gets warm.

Heat tape uses a resistance type switch to activate/deactivate them... Heat makes resistance which reduces overall voltage thru the circuit which is how it controls the temperature.

While I can see it being possible (with some uhh... modification engineering) to make a household thermostat work with a heating pad, UTH, Heat Tape, etc... but the thermostat would HAVE to physically be (for lack of totally bashing it apart, in which case let me konw and i'll just draw u up a schematic and a parts list for what u'd need) in the tank. But then still it would only sense the temperature around where it is.

If that doesn't make sense (cause i mighta just rambled on I cant' tell) let me know and I"ll give it a second shot. Right now I'm goin out to pickup my new 40BR WOOOOOOT

-Chris