Why is the rate of breathing in aquatic organisms much faster than in terrestrial organisms ?

The amount of dissolved oxygen is fairly low compared to the amount of oxygen in the air, the rate of breathing in aquatic organisms is much faster than that seen in terrestrial organisms. Fishes take in water through their mouths and force it past the gills where the dissolved oxygen is taken up by blood.

The breathing rate of aquatic animals is faster than that of terrestrial animals because the amount of dissolved oxygen in the water in much less than the amount on land, so they have to breathe more in order to get more oxygen.
Oxygen concentrations are much higher in air, which is about 21% oxygen, than in water.