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Are you sure it's a CAE? Cases of mistaken identity are common with these, which is why so many people end up with them totally unintentionally. That said, yes, they need large tanks all to themselves as adults. As juveniles, they can start fairly docile and then though ... watch out!! Watch some of those youtube videos suggested above and you'll see how big and ugly these guys get. They scare me.
Back to the cory--one thought. They really are a type of fish who need to live in groups to thrive. After you treat him with the medications recommended (I can't advise, never having had to medicate my fish myself ... *fingers crossed*), consider getting him at least four friends. I suspect he is stressed to the extreme and absolutely terrified without a school to "protect" him, so maybe that stress has somehow played into his physical condition. Humans comfort eat when they're stressed, right? So maybe fish do too ...