For the first time ever, I experienced a heater malfunction. The heater in my 55g somehow reset itself to a target temp of 91 degrees. I caught it at 87 while observing a cory swimming on his side unable to right himself. Inferentially I put my hand in the glass and it was hot! Opened the lid and a steam bath hit my face! Emergency water change!! Drained it to about 10% water left and refilled with 67 degree water. Tank quickly righted itself to 75 and all is well now! Whew!! That was close!