I find my cories are more active at two times, the first is after a water change, I can assume this is because the water is more oxygenated, and probably a little bit colder than the tank water. And second because the CO2 levels are higher, Cories get a lot of their oxygen from the surface when they cant get enough down at the bottom which would lead me to believe they are swimming around more and probably coming to the surface more because their is a deficiency of O2 in the water.
This is the point I was attempting to make.
Actually Cories can gulp some air from the surface, but they are not anabantoids, such as Betta and Gourami, that can breath from the surface. When the Cory gulps air, he swallows it, and it goes to his stomach. A small amount then can make it to his system. The rest does the same as air you swallow: it is released as gas through the intestine. This is what I was taught here. I did not research it, so I may be mistaken. It is true though that Cories do fart from swallowed air.
Nevertheless, Cories are river fish and like their water highly oxygenated. Some species will not thrive (they will die) in poorly oxygenated water.
In addition when conditions are not optimal the fish will look for better conditions while he is able, hence some of the activity, until lethargy sets in.
I do not know about co2 and o2 displacement, but I do know it has always been encouraged in this forum to add an airstone to Cory tanks. My Cories all love to swim and play where the water is moving rapidly and there is oxygenation.
Please correct me if I am mistaken in anything.
I think I would add an air stone or curtain where it is possible to give the Cories extra oxygen in some part of the tank.
