Yellow Tang problem ?

[GTP Guy]

Hey guys,

My yellow tang seems to have quit eating. It was fine one day then not eating anything the next, he used to come shooting over to where the food was and gobble it up with the rest of the fish. He even used to compete with our niger trigger for the Nori clip.

Now he barely swims, more floats in the current, and even when the food is right in front of him he just looks at it. He had lost some color so I added another air stone for oxygenation, his color came back but the attitude stayed, I changed food from brine shrimp to Mysis a little while ago but this happened before that. He must be eating something as he's still alive but I don't actually see him eating (even when I stand VERY far away LOL

Water Params:

SG:1.023
PH: 8.1
Phos: 0
Nitrates:5 PPM
nitrites :0
Ammonia: 0

Ideas ?