It takes a while to learn what a totally healthy fish looks like. I've been at this for 17 years and I still get fooled.
Unfortunately, many of us go through a lot of fish before we find those that survive. If you are feeding Garlic Oil as described in this thread
http://www.utahreefs.com/forum/forum_posts.asp?TID=34014 and have done everything you can to ensure a good healthy environment, there isn't much else you can do about this condition. The LFS may be right, but it's best not to assume.
Can you post a pic of the fish and of the tank just in case we can find something else?