Originally Posted by vickietory
I thought it might have ich because of the way it was darting around the tank. My experience has been that fish normally act this way before the white specs break out all over them.
A fish stressed from any number of things could cause it to dart about. Has the fishes fin color changed any more? gotten darker or lighter since the QT? If you could, a nice picture of the tang could go a long way towards someone determining what the issue may be.
(also, going to move this to the sick fish forum)