Breathing Fish Vs Human

While we observe fishes in an aquarium, the timing of the openings and closings of the mouth and gill slits are co-ordinated. 

If we count the number of times the fish opens and closes its mouth in a minute it is less than the number of times of the human breathes in and out in a minute. Since amount of oxygen present in the water is less compared with the air the acuatic animals breath faster than terrestrial animals.

Average respiratory rate reported in a healthy adult at rest is usually given as 12-18 breaths per minute.In normal 20° C water, the average gill movements per minute is 66.When water is at  30°C, goldfish increase their breathing to about 78 times per minute. So, the breathing rate of fish is also depends upon temperature.

Normal air contains approximately 21% oxygen, but the largest part is nitrogen or 79%.Water contains less oxygen per volume unit than air, but the oxygen concentration is in strong relation to the temperature of the water, pressure and the amount of dissolved salts.

The concentration of oxygen decreases with increased temperature (not in linear proportion). This is especially disadvantageous for animals of cold blood as metabolic rate and the need for oxygen increase with increased temperature. The concentration of oxygen per volume decreases with decreased pressure. This applies to air and water, the "thin air" at high altitudes being known to most people when climbing mountains. Similarly oxygen content should increase at increased depth (other factors being constant). The solubility of oxygen in water is less at increased salinity. This means that seawater contains less oxygen then freshwater at the same temperature and pressure.