My thinking is if the aquarium heater is not over sized, as in just sufficient, how could it overheat several hundred gallons if was stuck on? Maybe in an aquarium, but in a system with several hundred gallons or more I think it would be tough to really heat up the water to lethal range.
You are correct in thinking that Cecil.
In a perfect scenario, and there was absolutely no heat loss. You have 300 gallons of water and the temp is 80F. Let's say that 100F is lethal. With a 1000 watt(100% efficient) heater, your fish would be dead just on the inside of 15 hours. 30 hours with a 500 watt heater and 60 hours with a 250 watt heater.
The planets would have to pretty much line up for this to happen tho.
