I have one of those zoo-med rheostats and it is total junk. I can't tell any difference between having it set on the highest to the lowest setting to skipping the rheostat entirely and plugging the UTH directly in to the wall, so what good does it do to have it at all? From what I've read, I've gathered that the zoo-med t-stats have poor quality control, some work great and others don't, so I guess maybe the same is true with the rheostat and you and I happened to get bad ones.
If you don't want to spend the money on a t-stat, go to home depot or lowe's and get a light dimmer switch, which is a rheostat that actually works.
I usually really respect and agree with what Tim has to say, but in this case since I suspect the rheostat doesn't work properly, I'm not sure that switching what heating device it is hooked up to will do any good. I was using mine with a UTH during the brief time I used it before I switched to light dimmers then eventually to a t-stat.