In rivers, lakes, seas and oceans live more than 20 thousand different species of fish. Fish that live in fresh water are called freshwater, and those that live in saltwater are marine. Like other animals, fish live in warm water bodies and in cold polar seas.
Most fish are not able to breathe air, as gills can absorb only oxygen dissolved in water. However, up to the present time there are 3 species of fish that have lungs that allow them to breathe air. For example, the protopterus living in Africa can for a short time get out on land.
The fish moves in the water, cutting and relaxing the muscles. Also, it pushes away from the water, wagging its tail from side to side. And the fins are needed by the fish in order to change direction and maintain balance.