As I've described, the fish isn't showing any signs of internal parasites, though I can certainly try to check on that. I don't think I can give him any medications, as he isn't eating any sort of prepared food I could soak those medications in.
I'm aware that baby brine shrimp lose their nutrition quickly (though I don't think it's entirely gone within 12 hours), which is why I've been giving him newly hatched ones.
The recommendation for a better diet is interesting. "Live isopods", I've never heard of as a food source, and I wouldn't know where to get any marine ones to feed him. He's not eating the munnid isopods I already have in my tank, anyway. "LIve arthropods" is basically meaningless; that could mean anything from blue crabs, to adult brine shrimp, to butterflies. I'm already giving him live arthropods, in the form of freshly hatched brine shrimp. "Bloodworms" would be a freshwater food source, and one that might be too large for him. I'm going to try to get ahold of a live whiteworm culture, but I can't buy anything live online until probably September, as I live in Texas.
He did gain weight while in quarantine, is the thing. So, evidently, providing him with a significant number of baby brine shrimp works. Which is why I'm doing that now, in an isolation box.