Like I said, just trying to help you avert diaster. If you get a chance, you should peruse other emergencies- you can click on the tag or head over to the fish disease forum I think you'll fine one common trend- no one see's ich, velvet, or many other diseases "coming". Many reefers with ich in their systems can have healthy fish for years before a stress event lowers the fish's immune system and ich "shows" itself. I'm just trying to give the advice I would want to receive if I were in your shoes...I have had that tang In there for 6 months and he is doing very well no sights of stress I also have a 50 gallon tank as well for when he gets big.
What you do with that advice is up to you.