I'm considering switching my heat source from a heat bulb to a UTH. I was hoping someone could help me with some questions: - The instructions for one brand of UTH states that if the unit is mounted underneath the tank, the substrate cannot be any deeper than 2" or so. (The instructions, of course, are written assuming a reptile will be in the tank.) Since crabitat substrate must be deeper than 2", do I need to worry about overheating and/or stress cracking of the tank? - For a 20 gallon tank, a "small" size UTH is recommended. Do you think something that small will really heat the tank sufficiently? These heat pads aren't cheap, so if I buy one, I want to make sure to get the right size. - If mounted underneath, doesn't it get too hot when the crabs dig way down to molt? I would appreciate any input on this topic! Thanks!