Kole tang sick

[Jahalu]

I have a 75g mixed reef. Fish include banggai cardinal, firefish, clownfish, starry blenny, sleeper blue dot goby and a kole tang. It's a mature tank, all parameters are good. The tang has been in my tank for about 5 weeks. Acclimation went really well, no ich, no signs of stress. Fish has been happy pecking at sides of tank, and eating frozen mysis, frozen brine/veg mix, krill and formula 2 pellets. Two days ago he started hiding more than usual. I took a closer look and his right side looks very swollen, like he swallowed a marble that lodged on his right side. When his left side faces me, he looks completely normal. His skin is perfect on both sides- good color, no spots/sores. He is not eating. Any thoughts? I've searched online, but can't find anything that fits. Where did I go wrong? Is there anything I can do? Tank mates are all behaving normally.

 

[Pfisherman]

I've had bad luck with these bristletooth tangs. Folks say they are easy but I've done killed a kole and a chevron, both were eating for several weeks then started hitting skinny then hiding then kaput. Very saddening