I have a 40 gal for my normal and I have 2 UTH heaters and a CHE in the middle. I have the larger size for the hot side controlled on a thermastat and the smaller one for a 20 gal on the cool side controlled by a rheastat (the CHE is as well) this set up keeps the levels perfect 24 hours a day. 92 on the hot side, 82 on the cool side and the air at 80/82*.