I don't know if I may have purchased one that's too large for my tank, but I'm having the opposite situation; my tank is getting too hot for my liking. It's currently at 87, and the red light on the heater is STILL on even thought I set it to 75 to try and offset the temp fluctuation.These heaters won't run away and nuke your tank. They clearly are undersized anyways. They can't heat tanks it seems.