The pH value of the water is one of the most important factors affecting the organisms living in a pond, since they can tolerate only slight fluctuations in the pH value.
A pH value indicates the concentration of an acid or a lye in a scale extending from 0 to 14. Pure water with a pH value of 7 is considered neutral. Tap water generally has a pH value of between 7.0 and 8.0.
The pH value of alkaline solutions is greater than 7, that of acidic solutions less. The greater the divergence from the value of seven, the stronger the acidic, or alkaline, solution will be. Carbonated drinks will show an acid reaction, and a very strong acid, such as hydrochloric acid, will have a pH value of below 1. Soap solutions as produced by a typical household are alkaline and will have pH values of around 10.
The varied flora and fauna of a pond will thrive best in water where the pH values are in the range between 6 and 8.5. pH values outside this range could damage the fishes skin and gills, and in extreme cases could kill them.
As is generally known, the chemical formula for water is H2O. This means that in each molecule of water there are two atoms of hydrogen (H) and one atom of oxygen (O).
These water molecules break down into hydroxide ions with a negative charge (OH-) and hydrogen ions with a positive charge (H+). However, in aggregate the molecule is neutral.
The pH value is determined by the degree of concentration of the hydrogen ions. For reasons of mathematical convenience, a pH value is stated as a negative logarithm of the concentration of hydrogen ions. If, for example, the concentration of hydrogen ions is 0.01 mol/l (=10-2 mol/l), the solution in question will be said to have a pH value of 2.
The pH value is inversely proportional to the degree of concentration of the hydrogen ions. If the pH value falls from, say, 7 to 6, then the concentration of hydrogen ions increases tenfold.
Oxygen
Carbon Dioxide