1) Feed your tang more, he is real skinny.
2) Might try increasing shutterspeed
3) Decrease shutterspeed (good luck with this one on this fish) and don't use the flash Using the flash it will be difficult to get it's true color
4) Use an off camera flash over the top of the aquarium or down the side.
BTW, I thought the pic was pretty darn good for that fish, it's as good as any I've taken of Paracanthurus hepatis. It's very hard to get good color on that fish with an onboard flash, it washes the color out.