Yellow Tang problem ?

[GTP Guy]

I was feeding them brine (on recommendation of my LFS) but I was also feeding him Nori as well (I knew he was a grazer) and he was eating well, then he just stopped eating.

Signs of disease that I've seen:

Not eating
some loss of color
rapid breathing
his mouth never closes

There are no sores or anything that I can see externally, I looked at his gills last night and i couldn't see any redness but it was dark and I was using a flash light.

I'm starting to get worried about him as he's starting to get thin.