Fish have gills, which are permeated by a network of blood capillaries for gas exchange and absorption of O2, there is a lateral line to feel vibration in the water .. It is unlikely that we will have cartilaginous fish in an aquarium, so they have a hydrostatic swim bladder and hold the fish. By changing the pressure and volumes in the bubble, the fish changes the depth at which it is located. They also have auxiliary elements in the form of glands that secrete mucus for better movement in the water and fins to regulate the direction of swimming.
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