Aquatic animals breathe much faster than terrestrial animals because of :

Aquatic animals breathe much faster than terrestrial animals primarily due to the low concentration of dissolved oxygen in water compared to air. In freshwater, the average concentration of dissolved oxygen is approximately 0.01% by weight (10 ppm), which is roughly 50 times lower than the oxygen concentration in an equivalent volume of air [1]. While air contains about 20.9% oxygen by volume, the amount available in water is significantly limited by solubility factors such as temperature and pressure [3]. Because the oxygen content in water is so much lower, aquatic organisms like fish must pump a relatively larger volume of water through their specialized respiratory structures, such as gills, to extract the necessary amount of oxygen for survival [3]. This requirement to process more of the medium per unit of time results in a significantly higher breathing rate compared to terrestrial animals [3].

Sources

1. [1] Environment, Shankar IAS Acedemy .(ed 10th) > Chapter 4: Aquatic Ecosystem > Dissolved oxygen: > p. 34
2. [3] https://www.fao.org/4/ac175e/AC175E04.htm
3. [2] Science-Class VII . NCERT(Revised ed 2025) > Chapter 9: Life Processes in Animals > 9.2.2 Do other animals breathe the same way as humans do? > p. 133