My friend had a what looks like a 44 gallon community tank. Did a WC and forgot to unplug the heater. The next morning he found his fish dead, some gasping for air. Thinking it was the water quality, he was about to do another WC when he found the water felt warm and gave a tingling sensation. Realized the heater was cracked and was zapping his fish all night long. Sadly there were no survivors. =(