I have a yellow tang that I introduced a month ago. He came to me from a very established Lfs. He was in a tank there for 6 weeks. Fish has always eaten well and is still eating well. The last few days he rubs the base of his tail on the back glass non stop. At first I just thought he was chasing his reflection, but the base of his tail is showing redness. He does not show any spots or signs of ich. Any ideas?