I actually buried the submersible in my oscar tank(next to the UGF lift tube, so I'd still get water flow around it), but don't the Visi-therms have an overheat protection mechanism? I stay away from Tetra heaters after two stuck on failures with different heaters. I know it can happen to any heater, but my experience has been three heater failures, the two Tetra and the third(a Penn Plax, by coincidence) just plain died, but at least it was close to 15 years old and it didn't kill any of my fish.