Okay, the guy I got my gecko from says that I should not place an undertank heater on the bottom of this glass tank, but if I place it on the side the sand doesn't heat up properly. The tank is a 20 gal. long reptile tank, and I've got some white calcisand substrate. The tank is also on some small feet to keep it from forming a heat bubble. The heater is a Zoo-med 20 gallon. Is this configuration going to crack the glass of my tank?