Sniff, sniff I still am crying over my 90g tank. Last year I had an old heater that I should have replaced break in half and shock the tank.
My inverts were frozen in place ...dead. I lost almost all my corals $1500.00 worth by estimation; I have not yet begun to rebuild the reef. As my serpent star fish moved and traveled across the tank its arms fell off.
All the tiny critters in the sand were killed, to say my tank was off balance is an understatement. I didn't lose a single fish, but my nitrates were so high and nothing could bring it down but daily water changes.
In the aftermath of that disaster...my sump/refugium was made to try and fix things so I can again have corals...but that’s another story.
EDIT:
I now only have Titanium heaters and only in the sump.