powder brown tang not looking good

so i was feed my little hungry fish today and i watched my powder brown tang and it went to eat but could'nt never get the food in it mouth i start looking closer and its face has pulled back and its teeth are showing what could be wrong i have changed anything in the tank and nothing new been put in the tank but a sps coral in the last month what could be going on my tank is

125 gal

no2 0
no3 0
ph 8.2
mag 1350
sali 1.026
pho 0
amm 0
cal 410
phosphate 0
plz help

 

could be quite a number of things.
A) Try and grab a picutre if you can.
B) Get a qt tank set up which will be large enough for a tang.
C) The biggest question is what are its tank mates? I.e.e bullying, stress..

Could be a flesh eating disease or other type of bacterial infection. Skin receding sounds like flesh eating disease, but a pic can help id this.

 

It could just be the fish...I had a powder brown a couple of years ago...had him for months...and then he just died on me. I lost nothing else...and never found a reason. I have read that browns can be hard to keep...so I have steered away from them every since.