Kingskimmer, it would appear "out" is a good place for you to be.
Octopus, if I'm not mistaken, and maybe I am, there have been a few threads posted about this problem you've been having. Unfortunately, as was mentioned on the other threads, even if it takes tearing all your rockwork apart, you're going to need to catch that Tang. Reliable treatment of this parasite is going to take hyposalinity treatment for at least 8 weeks. Not only that, you need to catch ALL the fish and get them out of the display and treat them as well. In this time period, your display tank needs to remain fishless for the same 8 week period.
Rid Ich, Kick ich...these are worthless. Anything that will kill Ich, will also kill all the beneficial critters in your tank. They are NOT at all reef safe. Granted, they probably won't kill your corals, but they'll kill all the other good stuff in your tank.