Getting a 20 gal long All Glass Critter Cage tomorrow for my corn. I have read to cover 1/4 to 1/3 of the bottom with a UTH. So far no problems I understand that I get confused when it comes to wattage. The tank is approximately 30 inches long and 15 inches wide. That means my UTH would have to be 15 inches wide by 7.5 to 10 inches long or as close as I can get. The wattage on this size UTH seems awful high. Can any body help me out here. A zoomed 10 to 20 gallon UTH is only 6x8 inches and does not have wattage listed. Big Apple has a 13.8 in. by 7.9 in. Intellitemp Reptile heat mat that is 20 watts. I just don't know wich way to go on this. Any help would be appreciated. Thanks in advance.