Hey! I'm 7w into my purple tang. 6 weeks into ich treatment via hypo.
Here's the thread where I went through treatment, some pictures, videos and all the advice I got: http://www.aquariumadvice.com/viewtopic.php?t=46092
My fish was already in QT
when I noticed he had ich as that's where he went when he first came in (knowing their reputation). I started hypo on him about a week after I got him when I realized that he had ich for sure. He's been spot free for 4 weeks now and tomorrow I start the process of raising my salinity. He's done really well. I was also worried about him not having rock in there with him. He really prefers the rockwork to PVC
piping. I ended up putting base (so that I didn't have to worry about die off from the hypo process) rock in there with him and he swims all around it. It is so beautiful to watch. He never would have survived if I didn't treat him. It got pretty bad just before I got the salinity all the way down but he turned the corner to good health fast after that.
You might want a refractometer in house as well as a pH meter so that you can begin treating him fast if he takes a turn for the worse. Any chance of a picture? QS & Steve may see something there that affects what they tell you to do.
Good luck. It isn't a trivial purchase to have this happen to!