Naso tang turned black

[JamesChambers]

What does this mean? I just added him into my tank and he is grazing a lot on my rocks. There is a lot of algae in my tank and he is doing a great job but ever since I got him he has been mainly black. Sometimes he takes his spots out and turns a lighter grey, like right now, however, he usually turns straight back to black. Is this just him getting used to the tank or is it another problem? The weird thing is when I put anything into the tank, like something to move the sand around, he darts away and turns to his healthy light grey colours, even though I assume he is more stressed. If anyone can give me any possibilities as to why he's doing this then It will be much appreciated.

 

[Just_Poppy]

Hi, I just introduced my naso to the main aquarium after 4 weeks in isolation. He does change color due to stress, so when they feel comfortable mine anyways turns back to grey, the main body that is. When I was acclimating him, I am sure he was nervous as I moved him into a container and he was like spotted until I put it in the main tank. After this he returned to normal colors and is happy. They will also change color under the lighting conditions, but mostly under stress. What are your water parameters?