With my experiences owning an aquarium service company, I've had the great pleasure of working with some very large reef tanks, and this has definitely made me change my mind on a lot of things. After caring for an Achilles Tang (only about 6") in a 10 foot reef tank, I've come to the conclusion that Achilles Tangs really don't belong in 6 foot tanks. Not only do they seem to never relax in a 6 foot tank, but they also stress out other tank mates because they are SOOOOO active (and often aggressive too). The one in the 10 foot tank that I cared for was not quarantined (none of the fish were), and I never saw a spot of Ich on any of those fish in 2 years even though I know there was Ich in there. I'm quite sure that the tank size helped alleviate stress in the tank, and that in and of itself prevented disease.
Anyway, I don't recommend the Achilles for your tank. They have a really BIG attitude to go along with their really BIG activity level. For every recommendation you ever get though (on any topic), you will be able to find others that "did just fine". Your call.
