issue with naso ... help

[naesco]

Do you have a QT? If not you can feed the fish food soaked in selcon and garlic extract (garlic extreme).
You need to know that naso tangs require brown algae as part of their diet.
Have you had this tang for a while because generally they will not fight each other once they are used to each other unless they are crowded.
That is a lot of fish in your pic. Are you sure you are not overstocked and this is a water quality issue?
Have you lost other fish recently?