Hey guys, this thread is very old but it did help me.
I have a Naso that we have had for over 18 months and he got ill due to a PH problem as we discovered, he initially stopped eating, after we discovered what the problem was, and rectified it, he still remained unwell even though all the other fish were doing well.. 2 Blue tangs, a flame angel and 2 clowns.. He was on the brink of death, so we quickly set up our old quarantine tank to isolate him as the others were beginning to pick on him.
He began to show improvement although still not eating, he was swimming around again. After about 3 days in the quarantine, now 7 days without eating, we put him back in the main tank in a hope the others eating would inspire him!
It took another 3 days in the main tank before he did realise it was in his best interest to copy the other guys and he once again ate!!
He has been doing really well since and is back to his normal self..
