My thoughts would be that no happy healthy fish stops eating when food is available. With this in mind my first concern would be bullying, bullying to the point where it gave up eating but this would come with many transparent ripped fins, missing scales in my experience. If we rule that out we could question water conditions, but likely the others would show some sort of similar features. If the rest are eating voraciously, and otherwise active I would probably rule it out but you guys are the only ones who can monitor the water. Internal parasites can be nasty critters, frequently found in wild fish, and a potentially real problem even this long after the import imo. I find prazi to be effective and not very intrusive, I've used it pretty liberally with rays and had no noticeable issues. Aside from IP we could consider something like hexamita taking hold especially with the weird way it's arching itself. If you saw any stringy white poo it would make it the most likely scenario, and New Life Spectrum makes a pretty nifty food god hex shield that could help. Aside from these scenarios there are many more I'm sure, that I'm just not well informed on. Disease and treatment are not things I'm terribly informed on unfortunately, but these are the things that come to mind so I figured I'd share.