do not put a naso in a 90 gallon. even if it is 3". They get enormous. I have one right now that is about 14" and hes in a 200 gallon vat, and it looks to small for him. Nasos require atleast an 8 foot tank with 2 feet width (since they can easily get to 18" int he wild and possibly captivity)
i would say a 90 is also to small for an achilles. since these are harder them most tangs/fish in general they need a larger tank as well. They need A LOT of flow also...