Flexwatt or any heating element even a light can be controlled by a rheostat or a thermostat. But UTH will not burn out like lights, from the on off process of most stats. I use a biostat, and flexwatt for my small setup. Bottom heat may not be exactly natural, but as I have read too, it helps greatly with digestion, which is logical.
Now, on aquaria I DO NOT like the stick on, or most UTH. Because glass has odd properties. You buy a relatively small heat pad at one end, but the heat is not pulled across the glass in a gradient heat pattern. Temps drop off fast away from the heater on glass bottoms. So it may be 85-87 on the pad, but a 20 long may have a 68 degree corner if your house is cold.
When I sell a tank setup I try to sell a light for heat. I tell them DO NOT turn it on and off as this will greatly shorten the life of a bulb. A bulb burns 8-10 times the wattage the moment you turn it on, compared to the operating wattage. So I always tell them to get a lamp dimmer(rheostat) from Lowes/Home Depot. Not good for my bulb sales, but it is the truth. A light gives a great gradient heat pattern across a cage.
But lets face it, when you start collecting to many who has 50 tanks setup? So UTHs of various type are used. If you have a tank, and using one, I also recommend, a ceramic tile over most of the heat pad towards the cooler end of the tank. This allows a larger spread of the heat in a better pattern. IMO Also some stick on UTHs get to hot, that how I got my first female ball on a rescue, it had a bad belly burn. Again, the tile/slate/long flat rock helps greatly.
Now to answer your question specifically,, er,, what was it again?
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Thanks for reading.
Big Tom
www.herpzilla.com