They relax their sphincter muscles in the pneumatic duct that connects their gut with their gas bladder. By doing that, the air will be allowed to empty out of the bladder. This reflex action is sometimes called "gas-spitting".

they have an organ called a swim bladder. It is filled with gas and it keeps them afloat.

oxygen and carbon minoxide and bubble gas co2

gas bladder

The swim bladder is the organ that keeps the fish balanced and upright, is an internal gas-filled organ that contributes to the ability of a fish to control its buoyancy. It is also known as the gas bladder, fish maw or air bladder.