Sorry for your loss. I went thru the same thing- lost two clowns, an anthias, a royal grama, a mandarin, and a molly. Tank had no new additions for months, and no changes. Just one day clown was dead- rest of the fish looked ok. Next day the rest of the fish were dead.
One molly and a algae blenny seemed to not be affected and are actually still in the tank- I never added any more fish to that tank as I never went fallow so I assume it’s still in the tank.
I have a few tanks, and when I get around to it I’ll put those two fish in QT and let the tank go fallow.
good luck man and sorry again that happened.