The gas that fish extract from water is oxygen. Fish breathe by drawing water in through their mouths and forcing it over their gills, where oxygen is absorbed into their bloodstream and carbon dioxide is released. Gills are highly specialized organs that allow fish to efficiently extract dissolved oxygen from water, which is vital for their survival. The ability to extract oxygen exemplifies the adaptation of aquatic animals to their environment, enabling them to thrive in various aquatic ecosystems. This process is critical not just for the fish itself but also for maintaining the ecological balance of water habitats.