Depends on the UTH Ultratherm, Fluker, and Rzilla will not get hotter then 100 degrees. I have used all 3 and never once used a thermostat on them. My Fluker has been running 24/7 for almost 3 years now without a tstat and never had an issue at all.

What size tub are you using or is it a glass tank? If it's a glass tank cover up 90% of the top and the temps will go up all the heat is escaping out the top. No need to get a second tstat.