What people are trying to tell you is that you'll be wasting your time QTing your new fish because your display likely still has ick in it even though your current fish display no symptoms. Add a new sensitive tang to that tank and you could have a full-on outbreak that could cause your "ick-resistent" fish to get sick too. It could crash your whole system quite quickly.
Believe me, I've been there and it's enough to make you tear out your hair.