PlanetCatfish.com

I've often noticed that when I see sick or dying Corydoras catfish, they seem to spend an awful lot of time near the surface of the water, and in some cases gulp down air like it was going out of style. They also move much more slowly than is normal, and do not hang out with their shoal nearly as much.

Does this normally happen when a cory becomes ill, or is this just in my head?

It is not just you. Almost every cory I've had that died spent their last days doing exactly as you described. I just did a post actually about 3 of my fish who have had these symptoms (2 died, 1 is about to I think). They are possibly "normal" symptoms from not being able to get enough oxygen. What's causing them to not get enough oxygen, I don't know.