Yeah...us too? How do those loveable glup, glups breathe? And do they ever come up for air?
Fish use a specialized organ called gills in order to breathethe oxygen dissolved in water. The mouth and throat of a fishforce water over the gills. Gills have a lot of blood vessels. The water has a higher concentration of oxygen than the blood.
for more info: http://reptools.rutgers.edu/Functpage34.html