Explain how fish maintain buoyancy?

Most fish (especially the "Bony fish") achieve neutral buoyancy through a sac called the swim bladder in there body of which is filled with air, not much but it'a just enough to keep then a float. The fish can control this organ, and using that they can fill or release air to allow them to rise or sink.

Cartilaginous fish however, do not have a swim bladder.

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explanation:

weathering is the answer.