How is he doing now? I had a cory act the same way, I think he had a swim bladder irritation and it got worse until I finally had to euthanize him. He was never quite healthy since I got him, though.
I think I may have been too hasty, though. I've since learned that sometimes after over-eating, a cory can have too much food pressing against the swim bladder and that will cause them to look like they are dying, since they won't swim normally and end up upside down and all over the tank in odd positions. I know mine was ill before he ate and had the swim bladder problem, so I really don't know that he would have made it if I'd waited. I made the best decision I could at the time, trying to avoid passing along any pathogens to my other fish. I had another fish in my quarantine tank at the time who looked better so euthanizing was my only apparent option.
I have also lost cories after water changes before. I'm not sure why. I hope your cory will be okay. Please post an update.
