Every tang I have ever gotten no matter who from, had ich, but I have never lost a tang. However I don’t think that the “medicines” they sell for it works. It always came back.
I admit I have never used copper to treat it, nor had at the time, a QT
So in my reef...
Tangs need warmth, my tang stayed stressed as long as my temps were kept at 78, when I upped the temps to the 80s the fish acted better, and then I started using shrimp soaked in fresh garlic juice to try and boost the immunity. I also purchased cleaner shrimp which likes to pick the parasite off of the fish.
My tang managed to stay healthy long enough for the parasite to die off because it had no host. Now even a full tank move and the fish didn't get ich, whereas before it would get it even after a simple water change.
So if your system is infected and you can’t remove all your fish, because the QT is too small for all of them…Why not try feeding them some garlic juice laced shrimp, up your temps to the 80s, and get a couple of cleaner shrimp?
Warm temps does nothing to kill ich, it just makes the fish happier. Some swear by garlic, and some disagree, cleaner shrimp only pick at the white spot and not clean the parasite at the free swimming stage .However keep in mind it is the white spot stage that kills the fish.
In spite of all the arguments…this worked for me, and may work for you.