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What is pH?
The pH scale, ranging from 0 to 14, is a measurement of the acidity or alkalinity of a substance. A pH of 7 is neutral and nonreactive. Substances with a pH of less than 7 are acids and those with a pH of more than 7 are alkaline or bases. On a chemical level, pH is the measurement of the hydrogen ion concentration. The higher the pH, the fewer free hydrogen ions in the substance.
Healthy Water Systems
Pure water has a pH of 7. However, the dissolved chemicals, contaminants and aquatic life in a water system affect the pH. The nominal pH range for healthy aquaculture is between 6.5 and 9.0 because fish have a natural blood pH of 7.4.
Dissolved Carbon Dioxide
Water plants use carbon dioxide during photosynthesis and help balance the carbon dioxide fish give off during the respiration process. Some of the excess carbon dioxide will dissolve in the water. When the water has all the aqueous carbon dioxide it can absorb, the excess will form carbonic acid and lower the pH of the water system.
Environmental Effects
If the pH of the water system falls below 5.0, fish will be begin to suffocate. Even a slight decrease in pH in water stresses the fish because their blood cannot carry oxygen as efficiently in an acidic environment.
Ironically, high levels of dissolved carbon dioxide also contribute to very alkaline or "hard" water. When water with low pH because of carbonic acid percolates through limestone, calcium and magnesium are dissolved from the rock into the groundwater. This results in a net increase of pH and alkalinity in well water.