What is the most likely mechanism fishes swim bladder uses to increase a buoyancy and let fish rise top of pond?

Gas secretion: Fishes secrete gases, such as oxygen, nitrogen, and carbon dioxide, into the swim bladder. This increases the volume of the swim bladder, reducing the fish's overall density and allowing it to rise in the water. The secretion of gases is controlled by the rete mirabile, a network of blood vessels that surrounds the swim bladder.