I used to get ich every time I intro'd a tang. Sometimes they'd make it, sometimes not.
Then I finally decided to do what Consicentious Aquarist recommended and do a freshwater/methylene blue dip.
These aren't for the faint of heart, nor the inexperienced. You have to buffer the water up so the pH matches your tank, the water needs to be heated to match the temp, and it should be well aerated, I use a cheap powerhead I zip tied a venturi to for this.
To this, you add enough methylene blue to make the water very dark. We use a flashlight to monitor the fish while we are dipping.
As I prepare the dip, I'm adding water from the tank to the fish bag to get it acclimated to that.
The first time I tried it was with my black tang. My GF and I were freaking out dropping it in, but the fish didn't seem to care at all. It just swam around. We do the dip for 5 mins. No ich!
Over the weekend I got a larger tank set-up and we found a beautiful PBT that the fish store had just gotten the night before. It was well colored, fat and eating well, so we bought it. We did the methylene blue/FW dip for 5 mins and intro'd it. Never a spot of ich!
I do have good circulation, run ozone and barebottom, I also soak with garlic every meal when I have new fish, which I personally feel do help, but I feel the methylene/FW dips are what really do it.