First get the tang into the QT. I take water from the main tank to start the QT tank and then add fresh sw
mix to the main tank. There's less stress to the fish that way.
Next, check the water parameters in both tanks. Get the nitrates levels down. I'd suggest daily water changes of 10%-20% but more experienced members might have other advice.
You might want to read this article
to help identify.
I have had one outbreak of marine Ich and treated it in a QT with hyposalinity (slowly lower the SG
to 1.009 and keep it there for a month).
Tell us more about your setup. . .
How big is the tank?
What's in it?
What are all the water parameters (Ph, Ammonia, nitrite, Nitrate, etc)?
What are you using for filtration?