The flesh and bones of fish are heavier than water.
Fish have air-filled sacs called swimbladders inside their bodies.
Fish use this marvelous balloon to adjust their buoyancy , so that they don't have to work to maintain their floating level. The amount of air in the swimbladder is controlled by the fish. If air is added, either from the throat or oxygen from the blood, the fish rises in the water. If air is removed from the bladder, the fish sinks. Instead of lungs, fish have gills.