The stock tank deicer I use has its own thermostat. It turns on at about 35 degrees and off again at 40. The water never gets warm. It merely stays thawed.
Mine is too but being a licensed electrican I just don't trust it so I also have it on a timer. I've seen way too many of those thermostats fail on stuff so I just put mine on a timer since our winters are so much milder one or two hours of run time is all I need.