Gaseous exchange in fish takes place between the gills and the surrounding water. The gills are located in an opercular cavity covered by a flap of skin called the operculum.
The mouth opens and the ﬂoor of the mouth is lowered so that the volume in the mouth is increased and pressure is lowered, water then enters into the mouth cavity.
The mouth is closed and the floor of the mouth raised so that the volume is reduced this raises the pressure, forcing water over gills and out through the operculum.
As water passes over the gills oxygen diffuses due to concentration gradient (partial pressure) into the blood stream.
In the body tissues, carbon IV oxide diffuses into the blood (due to concentration gradient, and is transported to the gills and diffuses out into the water.