Naso Tang acting funny

[Delphinus]

I hope it's nothing -- but for what it's worth, my tang (Z. desjardini) started doing that too about 6 months ago (I've now had him about 4 years). All of a sudden he would startle super easy from things that never used to bother him. I'd find him all black and motionless in his little parking spot. First time I saw it I thought he was dead. Still kinda creeps me out, I try not to do sudden moves and so on when around the tank nowadays.