Achilles Tang's are probably one of the most energetic tangs available, they need lots of space and flow. I would say a 65 gallon is too small, even for a juvenile Achiles. They stress out really easily, even more so than Powder Blues, and I think lack of space can be a big contributor to that.
Having said all that, they are georgeous fish so I understand the temptation, just trying to help you understand the risk...
Hope that helps...