Oooh, I know which breeder you are referring to. I got a fish from them at a local fish show and it had a fish louse, plus some mystery virus that took down a chunk of my tank. Sigh. Looked healthy too, but needed more vigorous and longer quarantine than what I did.
Okay, good on the tap water. Unless it’s above 8.6 or so I wouldn’t personally adjust it, but did you ever check the hardness or if the pH was stable from day to day? I only ask because sometimes a high pH can still be volatile and swing quite a bit, which could be a stress factor.
This fish might need more days of metro, and possibly some additional treatment. Could you use your nail to lift his gill plate under bright light and tell me what color they are? Light pink like salmon, red like raw steak, or more maroon/brown?