I have a number of the Hydor Theos, I bought a bunch when my local LFS started carrying them.... from 100 watts to 400 watts....NONE of them will hold a consistant temperature. ALL of them will over heat the tank they are in if given the chance so I now will only use them with a separate controller, or for heating up tanks that are way oversized for the wattage ( I have a single 300 watt one set at 80 degrees that keeps my 600 gallon tank at 86 with the ambient temp of my basement in the low to mid 70's), and my reservoir/aging tanks which, even those I have to watch or else I'll have to wait for the temp to drop back down from the high 90's, no matter what temp they are "set" at. They do work good for heating up my reservoirs fast though....but you should see my electric meter spin.
Mike