I use "REPTITHERM U.T.H. BY: ZOO-MED" and I will never try another again. Others I have tried are the "EXO-TERRA" and the generic one that just says "UNDER TANK HEAT PAD".
The "Zoo-Med" heating pad will heat your pad up to 100F if you let it run solid, you NEED a thermostat controller for these things, and any mild ventilation under the tank.
Mine sticks directly to the bottom glass and the low profile form will let this fit under any, 30Gallon or higher, tank. My stand has two 1 inch holes drilled on both the right and left side, so that ambient heat can travel the length of the tank, while allowing concentrated heat to not rise to a level that would kill my herps. These things will shut themselves down after reaching 110F as a precaution, but this may ultimately destroy the heater as this is an emergency step not a normal operation method. I place my thermal sensor directly in the center of the pad, just underneath it (Taped to the heater). When the actual heating element reaches the desired temperature, the heater turns off. (70 for my herps) If you place the thermal sensor in the tank, the heater may heat to 100F in order to raise the temp up to its desired temp, which is bad... because that will cook your herps, and cause the heater to turn on more, hotter air cools quicker, and rises faster... if you only heat the element to 70, then the tank will stay at 70 longer... if you heat it to 100, it will be 70 for a few seconds, then cold air will drop, and push the hot air out fast. Any-way...
10G - 20G Longs will need to be propped up about 1/8 of an inch to stop the tank from resting on the heater connection underneath, possibly cracking the glass. (I am sure that the ones for 20Gallons and lower come with rubber risers, but I would head to a hardware store and purchase an additional eight more little rubber feet pads just to distribute the weight better.
My tank is a 55 Gallon tank with several creatures, and as soon as I build my back wall, there will be more insulation inside and out to help keep the heat regulated. As if the 20 pounds of stone that cover the bottom aren't enough... You can use this same trick with a heat rock, to make it herp safe. Use a small heat rock, (One that can get wet, or just coat it in fish tank caulking and sand for added appeal, I find heat rocks to be real fake looking and ugly... Then place the rock directly on top of the thermal sensor, and possibly burry the rock with bedding to limit herp contact. Usually the rocks turn off at 90F - 100F but you just set the sensor to turn the rock off when you want it to. Place a thermometer directly on the rock when adjusting the sensor, as most thermal controllers have only a useless Red-to-Blue color indicator or an even more useless 0-10 on the dial.