maybe being bullied by something else??
you have a couple yellow tangs dont you?? they are quite territorial especially when another species of tang is introduced so even though it was eating in quarantine when food is introduced around healthy yellows alot of the time its when this territorial behavior comes into play the fish are forced to confront each other for food or the latter happens and the weaker or newer fish gets stressed and doesnt eat or worse again if ich has been in your tank or around the tang its when it will show.moving the tang to quarantine would have added extra stress but not much else you can do. main goal now is to get the tang eating again and the only way this is going to happen is 2 ways....make it feel safe enough to eat something and entice its appetite.
by leaving the lights out and hands out of the tank lots of hiding places etc. youll make the guy feel like theres no danger so then it will graze for food, soak your nori in garlic extract to entice its appetite.its very very important now to keep good clean water and not to frighten him so he eats.
not much else you can do, if hes lethargic(i spell like crap lol) then your in for some work to bring him around but tangs are healthy eaters and if you can get him to eat again he can come around.if you decide to put him back in your display then i would suggest eithger moving around some rock or removing the tangs for a while untill the naso feels comfortable enough to swim and fend for himself.
sorry i cant be of more help.
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