There is always a chance that your tang does not have ick. But there is always a chance that it has nastier disease. Marine velvet could wipe out your tank within days. Flukes could spread to the rest of the fish in your tank and internal parasites like worms could spread in your system and slowly killing all your fish without you noticing. And you could have missing fish from time to time due to unknown reasons.
For long term success, I'd suggest everyone to QT. It's a pain but it's worthwhile. Now I am guessing that no other fish has been QT'ed before. Do a research on prazipro and if possible treat all of your fish with it. It help deworms the fish and also treats fluke. I believe it's a reef safe med but don't quote me for it, look it up.
I would just treat your naso with prazipro and observe your fish during the 7 days treatment and see if other complications arises. Since your main display might have ick already it MIGHT not be worth your extra time to do a full 6 weeks hypo. But at least keep it in your QT for a couple of weeks to avoid fast killing disease.
Steven
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