yes on strengh lost a saddle back today butterfly ( making it 6 fish now dead) with no external signs of a problem.
Looks like you've already tried CP, so I don't have any other suggestions.
I once had a 700 gallon QT with a group of fish that we just could not stop the mortality in - even histopathology didn't tell us what the cause was. After that, we just sterilized the tank (sigh), I suspect it was either microsporidians or a virus....but we'll never know.
Jay