As vertebrates, fish have these same needs for oxygen. Only instead of breathing it in from the air, they extract it from the water with their gills. This oxygen they are extracting isn't being torn from the water molecules themselves. They aren't pulling the the O from the H2O. The oxygen they absorb is 'dissolved oxygen', stable O2 molecules that become dissolved into the water through aeration and agitation, the same O2 that we breathe from the air.