Fish use the process of ventilation, moving the respitory medium over the resiptory surface, to exchange gases through its gills. Fish breathe oxygen gas that is dissolved in the water, not the oxygen in the water it self. Most fish use the motion of swimming and moving their mouth and gill covers to ventilate their gills. During this a current of water will flow in the mouth, pass through the slits of the pharynx, flows over the gills and lastly leaves the fishes body.