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 description 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, Tums, and brushing your teeth with baking soda. When acid like substances are mixed with base like substances, they react with each other producing some by-products 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 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.
Just knowing the pH doesn't give us the complete picture. A pH test of distilled water can show almost any value since just a tiny amount of residual impurities, either acid or base, can have a major effect on the ratio of the two.
For example; assume we have the equivalent of 1 acid component and 1 base component in our water (equal amounts means the pH is a neutral 7). Adding 100 more base components will cause a change of over 100 in the ratio (101 divided by 1) and cause the pH to go to just over 9. Now let's start with 1000 acid components and 1000 base components in our water; again equal amounts so the pH is still 7. If we now add the same 100 base components, the ratio changes only slightly (1100 divided by 1000) and the pH goes up just a little bit (to about 7.04). The Alkalinity of our water is related to the actual number of base components and can be thought of as the "intensity" of the pH. (There is also a similar measurement called acidity related to the number of acid components but since we are normally concerned with slightly base water, it is easier to measure the larger number of base components than the smaller number of acid components.)
If the alkalinity is low, it indicates that even a small amount of acid can cause a large change in our pH. Consider the pond owner whose pH was 8.0. He was told that 7.0 was better so he puts in chemicals to lower it. The next day, it is back to 8.0 so he adds more chemicals. The following day it tests at 7.5. He feels good because it is finally starting to come down and dumps in some more stuff. All of a sudden he finds that the pH is 5.0. His bio-converter bacteria were destroyed and his fish are dying of ammonia poisoning compounded with pH shock. Each treatment kept reducing the alkalinity until it was so low that the final addition caused a major pH transition.
Alkalinity is related to the amount of dissolved Calcium, Magnesium, and other compounds in the water and as such, alkalinity tends to be higher in "harder" water. Lime leaching out of concrete ponds is a primary source of alkalinity but it is also slowly increased by evaporation which concentrates the source compounds. Alkalinity is naturally decreased over time through bacterial action which produces acidic compounds that combine with and reduce the alkalinity components.
Alkalinity is most often measured in ppm (referred to as calcium carbonate equivalents). A measurement is normally made by pretreating the water sample with a pill, powder, or droplet solution which results in the sample turning blue. The alkalinity is then determined by measuring (from a calibrated pipette or by counting drops) the amount of a second acidic reagent required to change the color to pink. A recommended test kit should measure a range of 0 - 200 ppm. An Alkalinity test kit is recommended but not considered to be a requirement for the average pond keeper. In an established pond, the ideal Alkalinity measurement should be around 100 ppm. Readings from 50 to 200 are acceptable.
Much more important than either the actual pH and alkalinity measurements, assuming they are both in the acceptable ranges, are CHANGES to them. A typical established pond will normally settle down into an equilibrium state with a pH of about one half unit above or below the pH of the tap water used for replenishment. Over time (months), all of the inhabitants (bacteria, plants, and the fish) become acclimated to their environmental conditions. Stress occurs in all of them if they must adjust to any changes. Rapid changes in pH can cause extreme stress to the fish similar to shock in humans. A sudden change of a half or more pH unit in an established pond is an indication that something happened and the cause should be determined. Slow, longer term, changes provide other indications. Increasing pH and/or alkalinity trends in a pond are normally caused by lime leaching out of concrete and to a lesser degree by concentration due to evaporation and decomposing organic matter. Decreasing pH and alkalinity tendencies are primarily due to bacterial action that release acidic compounds. Concrete ponds usually stabilize at a slightly higher pH value than ponds with liners.
High alkalinity is normally prevented by routine water change outs (assuming the tap water has a lower alkalinity than the pond water). Increasing pH trends can be minimized by an initial pretreatment or curing of a new concrete pond. Fill the pond with water (no fish), add Muriatic acid (swimming pool acid) as necessary to adjust the pH to about 5. Circulate continuously and test daily, adding additional Muriatic acid to maintain the pH level until no additional acid is needed. This normally takes 2-3 days. After draining, the pretreatment cure is complete and the pond is ready to be filled for use (now you can put in some fish). A properly treated concrete pond will usually reach an equilibrium state where the production of compounds which reduce the alkalinity is matched by the components being leached out of the concrete.
Ponds with vinyl liners or of fibre glass construction tend to show a decrease in alkalinity over time and may need supplements to maintain an acceptable level. Raise alkalinity by adding Calcium Carbonate, concrete blocks, oyster shells, limestone, or even egg shells. To raise the alkalinity by 40 ppm, add 1/2 oz of Calcium Carbonate (precipitate powder) per 100 gallons of water. A bag of oyster shells or even a concrete block or two (not cinder block) submerged in the pond or filter area may be all that is needed. Keep a close eye on the pH while adjusting Alkalinity levels.
Established ponds will normally maintain their equilibrium pH value if sludge and decaying organic material is routinely removed from the pond, mechanical filter, and biological converter. Scheduled water change outs (10% per week for a small pond, less for larger ponds) are also helpful. Monitoring the pH by recording weekly readings (before the water change outs) can provide an excellent indication of any developing problems. pH values do change somewhat during each 24 hours, depending upon the temperature, quantity of plants (algae and others), and the size of the pond, so try to take the measurements at about the same time of day. Alkalinity measurements can provide a warning that a pH problem may be imminent.
If the pH gets out of control, high or low, increase aeration and conduct daily water change outs to bring it back into range. Recheck after each water change out and again in 24 hours. At a pH of 6 or 9, do daily 10% to 25% water change outs. For a pH of 5 or 10, do 25% to 50% water change outs. At pH extremes approaching 4 or 11, remove any remaining fish. CAUTION: Be sure and check and treat for any Ammonia presence BEFORE attempting to raise pH through either chemical or water change out means. Only under EMERGENCY conditions should chemical means be used to adjust the pH in a pond. Attempting to lower the pH chemically can be particularly hazardous to you, the biologic converter, and the fish (not necessarily in that order).
Repeating for emphasis, the value of the pH measurement, within the acceptable limits, is of little importance. A change, whether sudden or a slow trend, to the pH of an established pond, indicates action may be required and is why periodic pH measurements are important. Further, if your pH is reasonably stable and anywhere between 7.0 and 8.5, not only is there no need to attempt to adjust it, you probably will do more harm than good by trying to change it.