I found this, which explains the reasons behind this behavior (cory cats are members of the Callichthyidae family):
The callichthyids inhabit a variety of different habitats in the Neotropical region, from small, swift, oxygen-rich creeks to big rivers and flooded areas, including swampy and muddy habitats where oxygen might be virtually absent. To survive in these habitats, callichthyids perform air-breathing. The air is collected at the water surface and swallowed, since their "accessory respiratory organ" is the intestine, and is eventually expelled through the anus. In this family, however, the air swallowed plays a more important role in the maintenance of the hydrostatic balance than in respiration itself, contributing with about 75% of the necessary air for neutral buoyancy (Gee, 1976; Gee & Graham, 1978). Additionally, unlike the catfishes in the families Loricariidae and Trichomycteridae that practice aerial respiration only in case of hypoxia, the Callichthyidae breathe air continuously under all water conditions.
In other words, cory cats are constantly swallowing air and then farting it out. Watch them as they swim down from the top and you'll see the little fart bubbles rising.
