How big is too big

[Proteus]

I bought a Naso tang at a lfs. On the weekend. It was in a 30 gallon sales tank I thought it was a beautiful fish and I wanted a larger one as I have a 4 inch powder blue. But I wonder if it may be to big.

My tank is 180 gallon. 72" long. And the Naso is about 8".

What are your limits on fish size or do you think it may need a bigger tank. I really like the fish but want it happy.