achilles

[naesco]

Sal you have several problems.
Your tank is too small to accommodate one of the tangs you have chosen let alone three. Your newest naso is stressed out.
A clown tang is extremely vicious and is almost impossible to keep even by expert tang keepers. Naso's are also difficult but not impossible.

You need to get rid of, at the minimum, two of them.

In the short term move the naso to your quarantine tank. Put some rockwork in so that he can hid and feel safe.
Keep the lights off or dim.
Do not approach the tank except to feed her. You are adding to the stress.
Feed brown alge which is the natural food of this species instead of green algae.
Try soaking the algae and all other food with garlic extract (not juice) and selcon vitamins. Try also a little mysis and worms.

IME, the fish is stressed and will not eat in your main tank and therefore is starving.
It is also possible if this tang is from the Philippines or Indonesia where the use of cyanide is rampant, that it is dying a slow death from cyanide.

Thank you
Wayne