So you think it's probably NOT NTD? If that's the case, I'm really glad I didn't decide to just euthanize hm.
Well, what I was thinking about doing was putting the 3 neons in the big tank in the 10G with him. That way, just in case, if he does still have something contagious then it would only infect those 4 neons instead contaminating my big tank. If all seems well after a month or so then I may add a betta or something like that.