Ah a blue tang. I have heard and read that they are prone to all sorts of diseases. The guy who I traded with today who "has been keeping saltwater fish for 26 years" in fact advised me against tangs because, he claimed, they always carry ick. I would bet that you, OC disagree with that bit of wisdom.
I would indeed disagree on the 'always carry ick' comment.
I had 7 in all in that tank (started with 6 as 1/2 inch juvies to start and then the adopted adult one about 2 years later). Never had a instance of ich with them or any other saltwater fish I kept. As with freshwater fish, proper QT of new fish, and good water quality will keep away most issues.
I wish I had a photo of the fish when I got him. He lived 4 months at least in a QT tank. Ich gone within a few weeks, but secondary infections, fin rot, body rot, etc. took a long time to heal. He also suffered from 'Marine Head & Lateral Line Erosion.' Look at the Paracanthurus hepatus about halfway down in this link:
Marine Head & Lateral Line Erosion: A Description of the Syndrome and a Review of its Speculated Causes by Steven Pro - Reefkeeping.com
He was fed only 'goldfish' flake food in his first home, and once being switched to a proper diet, the lesions healed completely.
Being that he was the 'new' fish in the established set of 6, I was not certain he'd merge into the 'herd' as it were. He hid for a few days, but ventured out after about a week. He got chased away a lot at first, but they accepted him in the end. One part of his tail never healed completely, as the wounds were down too deep, so I could always tell him by his tail that looked like a chunk had been eaten off.
I am a HUGE believer in drip-acclimating fish (also plants, inverts, etc.). I think the shock of parameters we don't even test for can be very stressful to fish. That ich infested Tang was in water so salty, I'm surprised it was still alive. For over a year, the first owner had been adding new SALT water (never changing out water) to bring the water level up due to evaporation. SG was more than I could measure. I did over 2 hours of acclimating him to 'normal' water. I started with 2 gallons of his water, then added 2 gallons of my QT tank's water over 30 mins. Then I'd siphon off 2 gallons, and start again. I think if I'd just put him in 'good' water, it would have done him in.
For marine reef fish, I think a place to 'hide' during acclimation and QT is essential too. They can really get stressed if they feel vulnerable. Often, I'd just put a chunk of dead coral I had for them to feel safe, and keeping the light level down helps a lot.