For the record, pH is technically defined as the negative base 10 logarithm of the effective hydrogen ion concentration in gram equivalents per liter (whatever that means, and why is it spelled so pHunny?) We are going to skip many of the technical details since it is not really necessary that a pond keeper knows exactly what pH is. What is important is to know how to make pH measurements, to routinely make the measurements, and how to interpret their results. The discussion below is not intended for chemistry majors nor for anyone who desires a technically accurate deskzrgption of our subjects. It is provided for the average pond owner to visualize what is going on in his or her pond.
Various substances exhibit similar characteristics in how they react with other substances. A broad range of these characteristics has been divided into the two categories, acid and base, where the pH measurement is related to a ratio of the base components to the acid components. (We aren't going into what these components actually are.) If a substance has one acid component for each base component, it is said to be neutral and has a pH value of 7. Greater than 7 is less acid, more base, and less than 7 is more acid and less base. Each unit is 10 times the previous, i.e., a pH of 9 is 10 times more base than 8, a pH of 5 is 10 times more acid than 6. Some examples of more acid like things are vinegar, orange juice, and the liquid in your car battery that makes holes in your clothes. Bases include lye, antacids for your stomach, and brushing your teeth with baking soda. When acid like substances are mixed with base like substances, they react with each other producing some byproducts and leaving the resulting solution with a pH somewhere between the two original values. The further apart the pH of the two substances, the more energy is released in the reaction. Put a teaspoon of baking soda in a half glass of vinegar and see what happens.
We are familiar enough with the extremes of acids and bases to know it is not a good idea to place a bare hand in either battery acid or caustic lye, and we can assume that neither would be a good place to put our fish. A pH measurement will help us determine if our water is a proper place to put the fish. For our Koi ponds, the pH should normally be between 7.0 and 8.5, but it is probably acceptable to be anywhere between 6.0 and 9.0. Although most of the fish could tolerate a pH as low as 5.0, bio-converter bacteria are subject to damage. Long term conditions above 9.0 can cause kidney damage to the Koi.
Test kits are available that use drops, pills, or powders with a color chart to show various ranges of pH. A wide range pH test kit (Range 5.0 - 10.0) is considered as a requirement for all ponds. If higher accuracy is desired, one or more limited range test kits are nice to have for the ranges most often encountered. Battery operated, digital electronic pH meters are available that measure from 1-14 in 0.1 increments. Most of the inexpensive versions of these ($100 or less) provide readings that are both temperature and battery condition dependent. All require periodic calibration and the less expensive ones usually require calibration prior to each use. Since doing this calibration and maintenance of the meters with probe cleaning and storage solutions is more involved than making a chemical reading of the pH, an electronic pH meter is not considered appropriate for most pond keepers. Those who have difficulty distinguishing the small color differences of the chemical test color charts find them wonderful.