You might consider Hyposalinity. That does not stress fish, but you need a refractometer to be sure it is dead-on correct.
As was mentioned...once Ich is irradiated on the fish and in the display tank, it should not come back. Stress lowers the immune system, which makes fish more susceptible to various ailments ( not just Ich). Stress does not cause it to magically appear. QT future additions and you should be fine.
I disagree with the "its a small fish, so it will be fine for a while" concept. I'm not bashing you, but from what I have seen over the years, if you put a fish in a smaller than suggested tank, it almost always has issues. Tangs are a little different and happen to have more issues. You almost never see a "My Tang in my 120g or 240g tank has Ich" thread online. I'm not coming from a "its cruel" viewpoint, simply a " I've seen it hundreds of times, so if you choose to go that route, consider yourself warned" viewpoint.