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Why is the parameter called Dissolved Oxygen (D.O.) important for fish?
Dissolved Oxygen (D.O.) is the parameter measuring the amount of oxygen in the water available for use by aquatic life. The amount of oxygen that can be present in the water is directly dependent on the temperature of the water and can be affected by water mixing with air and the living creatures present in the water. Colder water can hold less total oxygen and deeper ponds of freshwater can have a thermocline below which the water has less oxygen. This is also because of the microorganisms at these depths exhaust much of the oxygen. Fish, other animal life and even plants and algae when they can't photosynthesize at night consume oxygen, so D.O. fluctuates throughout the day and night. During the day with successful photosynthesis plant life can add to the D.O. value. However it is possible to supersaturate the water with oxygen. This can also be mechanically produced by malfunctioning equipment like pumps or aeration equipment. Supersaturation can cause gas bubble disease in fish. Wind moving across water helps with gas exchange as any mechanical mixing of air with water. Waterfalls, fountains, bubbling diffusers or airstones all create current and mix air into the water which accelerates gas exchange. In general terms 5-15 ppm is good D.O., lower than 4 is bad leading to a collapse of respiration and higher than 25 is probably supersaturation leading to toxic oxygen effects and gas bubble disease |