Stress

[Dale]

Sounds like a nice set up
I also agree that a Q tank can be quite stressful as most are kept quite barren in case meds are needed. One advantage though is being able to run it dark (no stress to corals).
It sounds like you have acros but can you reduce your lighting period for a couple of days? Try running it dark when you are around and having the lights on for a short time when you are away. Or stick some cardboard on the the glass for a while (leaving some look through holes of course).
My experience with stressed fish is either they acclimate (sometimes slowly) or they give up the will to live and waste away quickly. The other thing to watch for is ich. It is an opportunistic parasite that can often show up in stressed fish.
A large blonde Naso is a beautiful fish - good luck again with him.