i have the same problem. i have a 20 gal long glass tank, and the substrate is on top of 1 single sheet of paper towel and just enough cypress mulch to cover the bottom, and i have to have the temp over a 100f for it to be atleast 80 on the coolside. i use 2 uth on each side. i tried all kinds of substrate but they all do the same. the probs to the thermostat and thermometers are all sitting ontop of the glass under the paper towel and cypress mulch. is there a substrate that lets the heat rise from the bottom to the top of the substrate. sorry to high jack, but i think thats what he is encounterting too.