Hippo tang with bad itch

[corper]

Yes thats right itch not ich. My tang keeps scratching himself on the same rock all the time, as well as on the gravel at the bottom of the tank. He doesn't have one spot on him, not one!

I was thinking that maybe I have a build up of copepods and amphipods. I been adding phyto plankton to the tank every three days to help get some natural food sources going. I'm wondering if their is already enough in the tank without my help and I'm setting things off balance. Too many little creatures for the fish to keep up with causing them to crawl on my poor tang and make him itchy.

Anyone have any thoughts or suggestions?