I have used a UTH(Under tank heater) in the winter successfully, it did a nice job of not heating up the substrate too much, while making the ambient air temp in the cage a few degrees warmer.
I would be cautionary about using it with a natural tank though, I now switched both of mine to vivariums, with thicker substrate etc, and I would be worried about too much heat accumulation under the tank and it cracking the glass. especially in my exo terra where I have a drainage layer and standing water in the very bottom of the tank.
You could avoid all that too and just put the UTH on the back wall of the tank, or side wall, granted there isn't a background on the back, but if there is put it on the side.
Mine is made by flukers, and maintains a constant 100F btw.
Overall though, if you keep your house within crested heat range, you wouldn't need to use one at all.
1.0 Red Ear Slider: Squirt
0.2.1 Cresteds: Maddie,Mocha,Caramel
0.1 Dog: Leia
1.0 Axolotl: Axle