My substrate's newspaper with slate in the corners. Yeah, I tried putting a second layer of slate over the hot part, but it was still at 100. It was at 106 with just one layer of slate(I just cleaned the tank and didn't put the second layer). I'll try putting a slate layers again, and if that doesn't work I'll get the rheostat.
I used duct tape to attach the UTH to the tank, could that be why it's overheating?