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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 5.5 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.

pH impacts fish in several ways:

First, if the pH is too low, a condition within the fish called “Acidosis” results. Symptoms are anorexia, and then production of excess slime, isolation, and resting on the bottom, finally, streaking of the fins, and death will occur. If the pH is too high, the fish will produce excess slime, and will gasp at the surface. Losses can be major. “Alkalosis” is hard to reverse once it occurs. On the other hand, Acidosis is rapidly corrected once the pH is brought up to a suitable range.

IMPORTANT: pH is an indicator that shows the toxicity of Ammonia. At higher pH values, ammonia is more toxic. Below pH 7.2 most Ammonia is ionized to “Ammonium” and is far less toxic. This has relevance if you are considering raising the pH in a system with accumulating ammonias.

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.

pH is prone to “fall” in un-buffered systems, and can fall precipitously due to Oxygen consumption, accumulation of Carbon dioxide, decay of fish and other wastes, and the normal activity of nitrifying bacteria which reduce Ammonia to Nitrite.

“Crashes” from a normal pH all the way down to pH 5.5 can occur overnight. At 5.5 the filter bacteria that may have contributed to the crash will shut down, preventing the crash from dropping yet further.

In systems where the pH has been chemically stabilized by any of the commercial buffers, the pH crash phenomena is not commonly seen.

You may already have noticed that the water pH even fluctuates during a day. For example, the reading is the lowest at the dawn but the highest in the afternoon. According to a study conducted by the University of Maine, the water pH will increase as the water temperature increases. However, the effects of photosynthesis on the plants life in the pond is far outweighed the effects of the water temperature. Plants give off more oxygen through photosynthesis than they give off carbon dioxide (CO2) through respiration. As oxygen increases and the carbon dioxide decreases, it results the water pH to go up.


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. 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.