Final answer:
Fish control their buoyancy by regulating the amount of gas in their swim bladder.
Step-by-step explanation:
Most fish control their buoyancy by regulating the amount of gas in their swim bladder. The swim bladder is a gas-filled organ that helps to control the buoyancy of the fish. By changing the amount of gas in the swim bladder, a fish can move up or down through the water column. For example, when a fish wants to rise in the water, it releases gas from the swim bladder; when it wants to sink, it takes in gas and increases its buoyancy.