Hey Caper, Yeah the same thing has happened to me aswell, twice , I put the heater in one of my tanks and it just stayed on and heated the water up and up, fortunately I noticed this so didn't put the fish in that tank. It also happend in our turtle tank, the thermostat broke and when we came back from holiday we saw the water BUBBLING ( ) and the turtles all together on the rocks. Hope you fish are alright