I set-up a new 20 long tank this afternoon using a T-Rex Cobra 10-20 size UTH as a sole source of heat on a timer for day/night control. It is taped to the underside of the tank with duct tape around the edges. The stand has an airspace of approx. 3" under the tank bottom for ventilation. For substrate, I used about 3/16 to 1/4" of playground sand under 12" square slate tiles. It took about an hour for the temperature to peak and hold steady at 90 deg. on the warm end with 80 deg. on the cool side. At this time, I see no need for a rheostat. We live in Florida with high summer temps and the set-up is kept in a warm room during the day - 78 deg. The Cobra UTH works great with gentle heat - they advertise that they are designed to raise the temp over the UTH by 10 degrees - as I have seen. This size UTH (10-20) is perfect for a slight rise in temperature for 20 long / 29 gal tanks. Hope this helps someone else set their leo home up.