I went though FOUR tangs for my 180g before I found one that fit in with the current fish. They all ate like pigs and looked to be capable of living in the tank. But none lasted more than four days. All the tangs I tried were naso tangs by the way. My water was fine, they were acclimated very well they just didnt make it. :-(
There isnt always "you did this wrong" issue....I believe alot has to do with how they are caught from the wild and where. I only had success when I added a white lined tang, (similar in looks to a mustard tang) who is still doing well in my 180g, six foot tank.
Keep in mind these poor fish are pretty much in transit for days before they get to your lfs
so they are pretty worn out by then. ALso, do you know when you lfs
got the fish in? I always wait at least a week sometimes more before I pick up a fish that has recently come into the store.
This is always a good question to ask the lfs
, no matter what fish you are buying.