Man, I just went through the same thing about a month ago. I had a Naso, 12", for about a year. I added a 12" Passer angel, and he kept eating like a pig for about 2 weeks after, then all of a sudden, hunger strike!!!
I read all I could. In my reading I found that for what ever reason, they (Naso's) will do this from time to time. It might be caused by adding a new fish, change in water chemistry, moving a rock, moving a power head, water temp, sick, ect....
My stopped eating and hung out in front of the Tunze fan for about a week. He stopped swimming and became VERY thin. I tried every Nori sheet (brown,red,green) out there, shrimp, clams, you name it I tried it. He had no interest at all. I was very careful not to over feed to much and make the water go bad. I also did 10% water changes evey week.
I tested the water everyday to make it as perfect as I could. Even hooked up another skimmer. I did use Selcon and put it directly in the tank, because fish do drink, so he would still be getting something of value. I did this every other day. Read the bottle on amount per gallon.
When I put in the Nori, I rubbed it with my fingers to make it very fine, then one day, (thank God) he ate a little. Then the next day a little more Nori, then some flakes....
Now he is back to normal eating everything in sight and swimming around just like he use to. His hunger strike last 10 days before he started eating a little. Again, thank God , when this started he was nice and fat.
Hopefully yours, won't last that long. I was very stressed. My Passer had flukes and the Naso was ticked off at something. Both have made it through.
