cyslyde,
i find with my achilles that frequent feedings are necessary to keep him alive...these fish are constantly on the move and use a lot of energy doing so. constant supply of greenstuffs is needed as well as fast water movement (achilles tangs live in surge zones of coast of Hawaii )
the most important thing with these or any other fish is making sure it is eating at the lfs, cause if its not eating there it won't eat for you either.
the max size is 18" according to literature...mine has grown about 1" since i got him last year, he's 5" now.
they can be very agressive to same size and shape fish, so introducing this fish last is the best way.
great fish by the way, with awesome coloration to boot
HTH