There are a few different answers here, depending on what type of fish we are asking about.
The most sophisticated types of fishes, the bony fishes, have a swim bladder. The fish can essentially inflate the swim bladder with gas from a special gas gland. The gas is basically just oxygen from the fish’s blood.
Bony fishes that spend most of their lives on the bottom of the ocean floor (flounder) don’t have a strong swim bladder, if at all – therefore, don’t float.
Lastly, sharks, skates and rays are all types of fish. But, they are more primitive. They stay afloat with a fancy liver filled with oil, using their long pectoral fins for balance in midwaters, and maintaining a light framework. The lightframework I am referring to is that sharks, skates and rays all have a skeleton made up of cartilage (the same stuff found in our nose and ears).
Image (c) wiki-fish.com. (Yellow Tang – a true bony fish)
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