Sounds to me the best thing for you and any fish you have is to get out of the hobby.It already was in the QT it’s probably he and the other fish won’t make it. I don’t have the time to deal with multiple QT tanks. So I took him out and put him back in the display. I dipped him with the fritz copper for two days he didn’t improve. I don’t think the velvet is going to go away without weeks to months of treatment. Maybe it sounds callus but I don’t have the time to sit and babysit a fish and work 60+ hours a week. So at this point I’ll keep feeding him and hope he pulls through. QT of 15g with a tang his size will require daily water changes with copper added and then if the other fish get sick I’ll have to do the same. I don’t have a large containment system for this type of scenario.