I guess we lucked out with ours and have never had a problem. We made sure it was eating at the store (ate pellets really well) and looked overall healthy. It was actually hard to get it to start eating frozen! lol. All he wanted were pellets. But now he'll eat anything. We made the mistake and put him in my dad's 125gallon when we took down our 150gallon. We set up our new 75gallon and sort of wanted him back, but I guess he's now my dad's "favorite" fish and there's no way he'd give him back .

So I made my dad buy us a little chevron tang as replacement! It's better anyway, the achilles is was too big for a 75gallon now anyway. He is the ruler of my dad's tank though. Gets along great with everyone including our old orange shoulder tang who's about 2-3" bigger than the achilles, but the achilles did have to show his stuff to the poor new scopas tang the first few days. Funny thing is the scopas is afraid of the achilles, but the achilles is the only one with marks on it from the fight! Go figure. The marks aren't too bad, just missing some of his orange on his tail, but it's healing quickly.
So my advice find one that looks healthy and is already eating (ask to see it eat!). Once settled, feed a good varied diet. My dad feeds a mixture of mysis, brine, bloodworms, rod's reef, garlic, and I think a coral food too (not that the fish eat that). Be careful about adding more tangs after the achilles, because they can be aggressive (but not as bad as my powder blue I just sold who even went after our new mandarin! I mean, come on, aggressive toward a little mandarin, give me a break!).
Old pic. He's much bigger now with a huge orange spot on his tail.
He used to be sooo little!
Our replacement and that evil powder blue! Got along with the chevron and our CBB fine (they were all added at the same time), but went crazy over the new anthias and mandarin.