I had the same problems.....all of them. I lost a powder blue, sohai tang, tribal blenny and 2 anthias. I was so frustrated till I went to my lfs
. He sold me a bottle of ich attack made by kordon. 100% organic and treats ich, fungus, protozoans, and dinoflagellates. I followed the directions and all my other fish got better and it never came back. It didn't harm my coral or inverts and I even had sps
in that tank. Its my understanding that ich is in every tank. Unavoidable but some fish are more tolerant or immune to it. Stress,temp swings, water quality, crowding....there's so many factors. Tangs are extremely common to be ich prone. Probably cuz they get crowded easily.