Naso Tang Sick?

[naesco]

If he has white spots on him and he is not eating you will not be able to feed him garlic soaked food
Set up a QT and medicate him with formalin or a marinefish medication that contains formalin. Note that formalin is carcinogenic and therefore be very careful in handling it and clean everything carefully.

The other fish in your aquarium can be treated with garlic extreme soaked dry foods, feeding very often and only garlic extract soaked foods.