Have a porcupine puffer about 7-8" long in a 160 with a handfull of tangs. No real aggression, but I see a quarter sized sore? On his back. It is white, and looks like his flesh is just melting away. Any ideas what it is? He mainly eats silver sides, so maybe malnutrition? 35 gal weekly water changes, params are good, maybe temp is a little cold. (75-76).