Explain how a submarine rises, falls, and maintains a fixed depth. Do fish use the same principles? (See The Buoyancy of Marine Animals, by Eric Denton, Scientific American, July 1960, p. 118, and Submarine Physics, by G. P. Harnwell, American Journal of Physics, March 1948, p. 127.)

Every submarine has ballast tank in it. This tank controls the flow of water in it thereby changing the density of the submarines. In accordance with the Archimedes principle, if the density of the object is more than that of the liquid, then the object will sink. If the density of the liquid is more than that of the object, the object will float.
If the submarine is about to float in the water, the ballast tank will let water out from its tank thereby decreasing its density. If it is about to sink, the ballast tank will let water to fill in its tank. This increases its density and will sink underwater.
The same principle is applied in the fish. When the fish is about to float, they expand their swim bladders and increase the buoyant force. They contract the bladder when they are about to sink underwater.