Apparently there is something wrong with my dimmer, because I tested it last night on Toby's tank (which has a heat lamp). As soon as I plugged it into Mona's dimmer, the light got REALLY bright, even on the lowest setting. I was able to adjust it from there, but it didn't go low enough - and immediately raised his hot side from 90 to 97. Yikes. So I tried to buy a new one today, but unfortunately they didn't have anything at the store... guess I'll just order one online, and keep her UTH on top until it arrives. Thanks everyone!