My sick fish isn't a Betta, but is a cousin - the dwarf gourami. He used to share a tank with my Betta females until he became aggressive, and now I can't trust him with my shrimp either (he started eating them).
The last shrimp he ate clouded the water really quickly and smells terrible. It killed all of my neon tetras and made the others a little sickly, but they got over it when the water was cleaned. I moved Matsya, the gourami into a large critter keeper so he was alone.
Now, Matsya lost his appetite a few days ago, but since he's skittish I couldn't get a closer look until I went to clean his water (full change once a week because of the lack of filter and smaller space).
The craziness that cost me my neons and a shrimp was about a month ago now, so unless its a parasite I don't know how it could be related. But..it was worth mentioning since they were store bought shrimp.
His illness has cause a huge abrasion looking injury, almost like scratches. I don't have anything in his tank that could hurt him so badly unless he rammed it incredibly hard over and over so I'm assuming its not that.
I put him in a pitcher to clean the water, I'll post the best picture I can get.