pH is a measure of the balance between acid and alkaline. People with swimming pools will also be familiar with this factor as the same scale is used for measuring pH in water. pH is measured on a scale of 0 to 14 on which pH 7 is neutral, less than 7 is acidic and more than 7 is basic (or alkaline). It is important for you to be familiar with pH as it interacts with many other aspects of water quality.

To understand pH it is necessary to learn a little about the chemistry of water. The chemical compound, water, is composed of hydrogen (H) and oxygen (0) and its chemical formula is H20. This simply means that each molecule of water is composed of two hydrogen atoms and one oxygen atom.

Water, however, actually exists as a balance between hydrogen ions (H+ ) and hydroxyl ions (OH-) Don't be too concerned if these terms are unfamiliar, they are simply names for the two components that make up water. The hydrogen ion (H+ ) is actually the acid component of water and the hydroxyl ion (OH-) is the basic or alkaline component of water, so that:-
  • In pure water these components are present in equal concentrations and the pH value is 7 or neutral.
  • When there are more hydrogen ions (H+) present and less hydroxyl ions (OH-) then the lower the pH value and the more acid.
  • When there are more hydroxyl ions (OH-) and less hydrogen ions (H+) then the higher the pH and the more basic or alkaline.

The pH of water is greatly affected by the amount of carbon dioxide (a product of respiration) in the water and the hardness of the water.


See also Alkalinity.
The optimum range for many species is between 6.5 and 9.0 on the pH scale. :
  • Barramundi 7.5 to 8.5
  • Yabbies (Cherax Destructor) 6.5 to 8.5
  • Silver Perch (Bidyanus Bidyanus) 6.5 to 9
  • Murray Cod 6 to 8
  • Rainbow Trout 6.5 to 9
  • Marron 6.5 to 8.5
  • Red Claw 6.5 to 8.5

The pH of the water should be kept as stable as possible as fluctuations in the pH will stress the stock, resulting in an increased risk of disease. If the pH fluctuates to an extremely high or low value then death of the stock may occur.


The pH of the water changes throughout the day.
As a general rule, the pH of the system water will be most acidic at dawn. This is due to the respiration occurring in the pond at night by all living things, producing carbon dioxide and using oxygen.
At sunrise, algae within the water starts to photosynthesise and utilise carbon dioxide. This in turn increases the pH of the water until dusk, when the algae stop photosynthesising and the pH drops again.
The higher the alkalinity of the system’s water, the lower the fluctuation of the pH within the system due to the buffering effect of the alkalinity.

Signs of Sub-optimal pH:
  • increase of mucus on gill surfaces
  • damage to eye lens and cornea
  • abnormal swimming behaviour
  • fin fray
  • death
  • poor phytoplankton and zooplankton growth.

Effects of Sub-Optimal pH:
  • stress
  • increased susceptibility to disease
  • low production levels
  • poor growth.

Causes of Sub-Optimal pH:
  • acidic water and soils
  • poorly buffered water, i.e. low alkalinity (<20 mg/l)
  • make up waters having high alkalinity and low hardness

Sub-optimal pH - What can I do?

To decrease high pH:
  • flush the system with lower pH water
  • reduce feeding rates to lower nutrient input and plant growth
  • Add small amounts of sodium bisulphate
  • add gypsum (CaSO4) to increase the calcium concentration
  • add alum (AlSO4) for immediate reduction of pH to avert imminent fish mortality.

To increase pH:
  • Add small amounts of sodium bicarbonate (bicarb)
Note add small amounts to ensure you do not go too far the other way