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Water Quality Management (Chemical Factors)

Water Quality Management (Chemical Factors)


  • Water is slightly alkaline in condition, with the optimal range of 6.5 - 8.
  • Less than 5 and more than 10 pH are lethal to fish.
  • The pH of pond water undergoes a diurnal change; it is alkaline during the day time and slightly acidic just before day break.
  • The fluctuations of pH are similar to dissolved oxygen. pH fluctuations are more in phytoplankton and weed infested waters and water with less hardness.
  • The difference in pH from morning to evening should not be more than 0.5. When pH increases, ammonia and nitrites become toxic, when it is reverse H2S becomes more toxic.
  • pH below 6.5 and above 8.5 is responsible for reduction of growth and resistance of parasitic infection increases in acidic waters.
  • Whenever pH falls, lime should be added to the pond water.
  • When pH is high, lime and urea should not be used to reduce pH. This is because NH3 becomes toxic at high pH. It is always better to add new water to maintain an optimal pH. Alum or aluminium sulphate can be used to reduce the PH and turbidity.
  • Alum removes phenolphthalein alkalinity; 1 ppm alum reduces 1 ppm phenolphthalein alkalinity.

Dissolved Oxygen:

  • Dissolved oxygen is one of the most important chemical parameters, which has a great influence on the survival and growth of fishes and prawns.
  • The pond water gets oxygen mainly through interaction of atmospheric air on the surface water of the pond and by photosynthesis.
  • It is produced only during day time and reaches a maximum at 3 PM, then gradually decreases up to early morning.
  • During night period, it decreases and it reaches a minimum during the early hours of the day. It is due to nil production of dissolved oxygen at night and instead, consumption of oxygen by plankton, weeds, fishes and prawns will be there.
  • During overcast days, the production of dissolved oxygen during the day is less and during the subsequent nights it decreases drastically.
  • When water temperature rises, oxygen is released into atmosphere. When salinity increases it is dissolved in water.
  • The optimum dissolved oxygen is 5-8 ppm. If less than 5 ppm the growth rate of the fish and prawns decreases and are prone to get diseases.
  • Less than 1ppm of dissolved oxygen results in death and more than 15 ppm results in gas bubble disease in fishes and prawns.
  • Whenever the animals are under stress due to less dissolved oxygen the food consumption temporarily decreases.
  • When oxygen decreases, fishes come to the surface and engulf the air.
  • Precautionary measures should take at nights, especially during the early hours to increase oxygen levels.
  • If it is very less, the water surface should be disturbed by beating water with bamboo poles or by running boats or by using aerators.                              


  • Alkalinity is caused by carbonates and bicarbonates or hydroxides of Ca, Mg, Na, K, NH4 and Fe.
  • Alkalinity is less in acidic soils and in ponds with more organic load.
  • Alkalinity is more in clay soil ponds and is increased if water is not exchanged.
  • The optimal level of total alkalinity is 40 - 150 ppm.
  • Alkalinity has direct effect on the production of plankton.


  • Hardness is caused by Ca and Mg.
  • Water with less than 40 ppm is soft and more than 40 ppm is hard.
  • The pond water with a hardness of 15 ppm or more is satisfactory for growth of fishes and do not require additional lime.
  • If water has less than 11 ppm hardness, it requires liming for higher production.
  • If it is less than 5 ppm, the growth rate is affected and causes eventual death of the fish.

Carbon Dioxide:

  • CO2 is produced during respiration and consumed during photosynthesis.
  • CO2 is less during daytime and more at nights.
  • The optimal level of CO2 is 5 ppm.
  • At high CO2 levels, pH decreases and water becomes acidic.
  • CO2 is accumulated in the blood of the animals. Then, the animals become sluggish, loss of resistance occurs, they cannot utilize dissolved oxygen and they ultimately die.
  • Whenever CO2 increases lime should be added to the pond.
  • 1 ppm of lime reduces 0.9 ppm of CO2.

Dissolved Ammonia and Its Compounds:

  • NH3 is found in excreta and is also released due to decomposition of organic matter and also increases with unfed feed due to high protein levels and death of phytoplankton.
  • It is an important compound influencing the growth of phytoplankton in the aquatic ecosystem.
  • The optimal limit of NH3 is 0.3-1.3 ppm and less than 0.1 ppm is unproductive.
  • Whenever NH3 increases pH also increases, but dissolved oxygen decreases.
  • CO2 reduces the toxic effect of NH3.
  • NH3 accumulates in the blood and oxygen transport in the blood reduces.  Gills become black, biochemical tissue is damaged and gaseous exchange is affected.
  • NH3 levels can be reduced with good management like no excess feed, optimal stocking and water exchange.
  • Lime should not be added when NH3 is high.
  • Optimal level of nitrites is 3.5 ppm.

Hydrogen Sulphide:

  • H2S is produced in anaerobic conditions by the action of micro organisms on sulphur compounds.
  • H2S is toxic to fish and prawn and is responsible for respiratory problems.
  • It should be less than 0.05 ppm in pond water.
  •  When H2S increases, lime should be added.
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