Sorry for your loss. Unfortunately, we can't alway's determine what causes us to lose our beloved finny freinds, I recently lost 15 mumichogs in two different 55 gallons in a matter of day's. Wipped out my stock of them completely. The bluegills, channel catfish, crayfish and pleco in one tank are fine, the white catfish and pumpinseed hybrid in the other are fine. No more mumichogs though. That was 15 in each tank, btw.
Once again, sorry for your lost.