have you been cleaning the glass? Sometimes the glass cleaner can get into the tank and end up killing the fish. Not telling you how to do anything but just was a thought.
I remember the pacu... I can only imagine the amount of grief this must mean for your family. What a long and amazing life he had though. It's just so upsetting when bad things happen that make no sense, heaters or equipment failing, but clearly you're both amazing fish keepers. A 40 lb pacu... really amazing. I'm very sorry for your losses.
wow, to have a pet for 28 years and go through so many memeories togetherand then to lose it so suddenly has to be very shocking, to say the least. my sincerest condolences to you.