You know when your Corys rush up to the top of the tank to get a gulp of air? The used air is expelled out the anal port. The Skeptical Aquarist says "Every so often your Cories will make a dash for the surface to gulp air. A highly-vascular section of the intestine serves as an accessory respiratory organ. The spent air, mostly exhausted of its oxygen, is expelled from the anus. In Corydoras, the swallowed air plays an even more important role in hydrostatic balance, keeping the fish neutrally buoyant."
Most of my catfish fart all the time, especially if its the morning after the weekly pellet feeding. With the bigger ones you can here them fart from across the room