I have had ich in my tank for 5 months. I haven't removed the fish because I do not want to break the tank down. Nothing has died yet (except for the one that added ich to the tank) but I am religious in keeping everything as perfect as I can. I also swear by garlic and other vitamin additves for the food and my neon gobies and cleaner shrimps have to get some credit.
Ok thats my history. I do see my tang and blenny from time to time slam against the rocks to scratch their bodies. I think it has to be ich causing them to itch even though you can't see it.