A few weeks ago I noticed one of my corys was swimming erratically, and I saw him kind of try to shoot up to the surface for a gulp of air and drift back down upside down. I figured he was a gonner, but he seems to be mostly better. When food falls, he actively roots in the gravel, but when he's just sitting still he tend to lean heavily to one side like he has problems stabilizing himself. I have 2 other cories that are the same species and they seem fine. I've only had them for a couple months, so I guess he may still be sick from when I got him. I originally bought 4, but one died in quarantine.
Anyone have any ideas what it could be/if there's any way to treat it?
Thanks!
Josh