Bit of a headbanger this one ,
Sometimes when things don't add up we tend to miss the obvious, too busy looking for something that isn't there.
It could be stress as you said mate but to effect everything like that at once not sure about that.(not writing it off)
That's why I was thinking poss parasite.
Think it's a case of illumination.
Parameters seem fine .
Temp seems fine
I'll contact my friend in the states see what he thinks ,might take a few days or so to get a reply but every little helps.
Just hope it's not to late for what you have left.
Quarantine tank seems the way ,if you can setup one for each fish separately (big ask I know ) but new clean water might help and float and drip acclimatise them.
Gives you a chance to inspect each fish separately for behaviour, and look carefully (which you probably already have) at each fish for signs ,and treat each one.
I know what it's like to lose things (I lost Percy was gutted) I really hope we can all help you find a answer Ratters before any more loss.
Will let you know soon as I hear back from friend in states.
Worst case senario would be total drain and clean tank .