I left the tub on top of the PVC cage for a while....the hot spot was a perfect hot spot. Ambient and cool spot were good also. Man...talk about a 2 for one deal. However....looks like I'll have to get something to make an air gap between 2 cages. I really don't want the lower cage working into the upper too much. I'll have to find something that looks decent also. Or some shelving that keeps the cages apart.