article: Respiration of fish
Fish breathe as land animals, but use the oxygen content in the medium in which they live, the water, instead of taking it directly from the atmosphere as gases. The organs of respiration are the gills of fish, protected under the operculum, opercular bones formed by and contained in a gill chamber. The water enters through the mouth and leaves dall'opercolo after passing the gills, thin filaments that function similar to that of our lungs. In fact the thin membrane that covers the gills to absorb oxygen by a dense network of blood capillaries, and from them to put into circulation throughout the body. At the same time the absorption of oxygen is the expulsion of carbon dioxide from the blood. The gills are highly efficient: it is estimated that about 80% can extract oxygen dissolved in water. In addition to the respiratory organs, the gills have an important role in maintaining just the right balance of salts. It 'obvious that the aquarium should keep well-oxygenated water.