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 (often called the total 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. 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.
Treatment: 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. 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 a few test 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. An alkalinity stabilization Apill@ can be made from plaster of paris. Just mix water with the plaster of paris, let it harden, and put it in an area that receives good water flow across the surface. A one pound plaster of paris Apill@ for each 500 gallons should suffice. Periodic replacement is necessary as they dissolve.
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 on the high side, conduct daily water change outs to bring it back into range. Recheck after each water change out and again in 24 hours. Don=t forget to check the pH of the water being added, it may be part of the problem. At a pH of 9, do daily 10% to 25% water change outs. For a pH of 10, do 25% to 50% water change outs. At pH extremes over 10, remove any remaining fish. Only under EMERGENCY conditions should chemical means be used to lower the pH in a pond. Any attempt to lower the pH chemically can be particularly hazardous to you, the biologic converter, and the fish (not necessarily in that order).
A low pH problem, below 7, is normally only observed in a liner based or an older concrete based pond. It is usually the result of a ApH crash@. This is when the total alkalinity has been consumed by the biologic activity in the converter and the pH suddenly drops. This has been observed to go as low as 4.5. Again, start water changeouts and increase aeration. Sodium bicarbonate (plain old pure baking soda, Arm & Hammer or generic store brand) will raise the pH and also increase the total alkalinity.
CAUTION: Be sure to check and treat for any ammonia presence BEFORE attempting to raise pH through either chemical or water change out means.
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 is 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.