Ammonia is my primary culprit (aside from the virus possibility aforementioned). I’m assuming you’re monitoring with a ammonia alert badge though?
Without treatment it is hard to say. Internal infections, uronema (internally), velvet (can stay in the gills and kill before you see it on the body), are other possibilities. You don’t see many fish recover after this point, unfortunately. I don’t think I’d add copper now, I’d just keep water quality high and stable if you don’t see evidence of other ailments and hope for the best.