I have two 200 watt eheim heaters that are turned to 79'F. They are hooked to my neptune system which shuts off the heaters when the temp is over 78'F, and turns on the heaters when the temp is below 77'F.
The heaters back each other up, and the neptune prevents over heating by heater failure.
BTW, I had a fluval heater blow up while i was making water for a water change.. I came back in the room and smelled smoke and fouled the water up.. had to throw out all the water . Would have been very bad if it was in my tank.