Okay FIRST off, YES you can use then on a tub.
You want to make sure you a properly regulating it.

Second an UTH is not meant to heat ambient temperature and I would bet you are not properly measuring your temps.
You need to measure the temperature on the inside flood of the enclosure above the UTH. the surface temperature is what you are regulating and if you are running wide open then it is well over 110 to 120 degrees. This is a fire hazard.