If he's in a 55 gal then you need 2 thermo hygros with probes. So the temp on the cool side is what he likes right now. He will go to the hot side if he gets too cold. I know when I was getting temps just right my girl buried herself on the hot side. Right over the uth. Scared me because I thought she was going to get nasty burns. Don't be frustrated. You will get it right. I promise. Relax a little, he just likes the cool side right now, or maybe the hide is better to him.