There are a few different answers – depending on what type of fish we are asking about.

The most sophisticated types of fishes – bony fishes – have a swim bladder. These fish can inflate their swim bladder with gas from a special gas gland. The gas is basically oxygen from the fish’s blood. Bony fishes that spend most of their lives on the bottom of the ocean floor (e.g., flounder) don’t have a strong swim bladder – therefore, don’t float.

Sharks, skates, and rays are all types of fish. However, these fishes are more primitive – lacking bones. They stay afloat with a liver filled with oil. They use long pectoral fins for balance in mid-waters while maintaining a light framework. The ‘light framework’ is made up of cartilage (the same material found in our nose and ears).

Image (c) wiki-fish.com. (Yellow Tang – a true bony fish)

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