So they were all fine and symptom free and then this diverse group of fish all suddenly died on the same day 1 or 2 days later of a disease.These fish have been showing signs of illness (disease/parasite/bacteria/etc) going back to at least December 2019. I did not read further back in the OP posts to see if illness symptoms were present longer than that.
Was this the cause of this mass death? Unknown, but it’s certainly high on the suspect list, if not the #1 suspect. It’s the only potential cause that has factual historical data to support it.
I don't think that is likely.