The pH of pond water affects fish, and some pond owners have acquired some type of device to measure the pH of their pond water. Measuring the pH is meaningless unless one knows the acceptable pH range for fish and a way to mitigate unacceptable pH of pond waters.
The pH is likely the most misunderstood by pond owners of all commonly measured water quality variables. Chemically speaking, pH is the negative logarithm of the hydrogen ion activity. This will not mean much to most pond owners. However, in plain language, pH is an index of the intensity of the acidic reaction or the basic reaction in pond water. A neutral water (neither acidic nor basic) has a pH of 7.0. The intensity of the acidic reaction in water increases exponentially as the pH falls further below 7.0. The basic reaction of water increases exponentially as the pH rises further above 7.0. Each unit change in pH (a change from 7 to 6 or 7 to 8) causes a 10-fold increase in the acidic or basic reaction, respectively.
It is important to note that a basic reaction often is called an alkaline reaction. However, I am using the term basic reaction to avoid confusion. There is another water quality variable known as alkalinity or total alkalinity. Alkalinity is related to pH but is not the same. Alkalinity is also essential in pond water, but I do not want you to become confused and think that the basic reaction of water is the same as its total alkalinity.
Fish can do well across a rather wide range of pH. Growth and survival are not appreciably influenced by pH's of 6-8.5. Moreover, excursions of pH to 5 or 9 for a few hours occasionally or even daily usually are not problematic. In my opinion, the ideal pH for most species is 7-8.
On the other hand, pond waters vary greatly in pH. In certain areas, pH naturally may be 2-4, and in some other areas, up to 10 or more. Drastically low pH occurs when the soil in the pond watershed or the pond bottom contains iron pyrite, which oxidizes to form sulfuric acid. Very high pH values result from highly alkali water in arid regions. Ponds with naturally low pH can usually be remediated by liming. Nothing practical usually can be done for alkali pond waters.
There also are ponds in wooded areas with significant inputs of humic and tannic acids from vegetative remains. Such ponds have a pH in the range of 4-6. This aberration also can be corrected by liming.
Liming typically is done by applying agricultural limestone. The limestone dissolves, increasing the bicarbonate concentration in water. Bicarbonate acts as a buffer in water by neutralizing acidity. The concentration of bicarbonate (and carbonate where pH is above 8.3) is measured as the total alkalinity mentioned above. Liming can effectively increase total alkalinity to 50-60 mg/L, and it usually is recommended for pond waters with less than 20-30 mg/L of total alkalinity.
A pond with a suitable total alkalinity of 20-30 mg/L minimum will typically have a pH in the early morning in the range of 6-8. Some pond waters have a natural alkalinity of 100-300 mg/L; fish will not suffer from such alkalities.
Ponds contain phytoplankton and sometimes higher aquatic plants. These remove carbon dioxide from the water in daylight for use in photosynthesis. These plants also respire and release carbon dioxide into the water constantly. In the daytime, more carbon dioxide is used in photosynthesis than is released by respiration, and the pH rises. At night, photosynthesis stops, but respiration continues, carbon dioxide accumulates, and the pH declines.
Knowing the pH helps decide whether to amend the water or not.
The pH of pond water is usually a function of the carbon dioxide concentration and total alkalinity. Put simply, carbon dioxide is acidic in water, and as plants remove carbon dioxide, the pH increases. Typically, pH is lowest about dawn and greatest in mid-afternoon. The more phytoplankton and other aquatic plants in a pond, the greater the pH rise during the day. Of course, the rise also depends upon the amount of light. The pH will not rise as high on an overcast day as on a clear day. In summer, on a clear day, the pH rise in ponds with the same amount of photosynthesis would be more significant in ponds with low alkalinity than in those with higher alkalinity. This is because alkalinity buffers against the amount of pH change.
The pH in surface water often rises above 10 for a few hours in the afternoon in fertilized ponds with alkalinity values of 5-15 mg/L. Of course, pH may exceed 9 even in ponds with plenty of alkalinity.
The pH decreases with depth below the surface in ponds because light intensity decreases and lessens the rate of photosynthesis. The upshot is that water pH in a given pond with acceptable water quality will change continuously during a 24-hour period. Also, the amount of daily change in pH will vary with phytoplankton abundance, amount of total alkalinity, amount of sunlight, turbidity of water (turbidity reduces light penetration), and seasonal and daily temperature changes.
Provided low alkalinity and low pH waters have been limed properly, the daily fluctuations in pH are of little significance, and they are natural occurrences to which fish are adapted. The pH is critical in determining if the pH of a pond should be adjusted by liming or if a pond is so alkaline that fish culture will be fraught with difficulty. Otherwise, the measurement of pH is not usually critical in pond management decisions.
A few additional comments are in order as follows:
- There are various means of measuring pH, but the color change method in response to pH indicator strips is not very accurate. The best method is with a pH meter; the small pocket pen-type pH meters can be reasonably accurate if cared for and used properly. If samples are to be sent to a laboratory, they should be held in the dark and analyzed as soon as possible to avoid pH changes.
- The total alkalinity is typically expressed as milligrams per liter (mg/L), which is the same as parts per million (ppm) of equivalent calcium carbonate (CaC03). This index reveals how much acidity the bicarbonate and carbonate in a sample can neutralize. Some alkalinity kits may use other units instead of parts per million (ppm) for alkalinity. Such kits will typically explain the units' relationship to parts per million. In other words, read the instructions if you use a kit to measure alkalinity. The best approach is to send a sample to a laboratory.
- If ponds have a total alkalinity above 20-30 mg/L, they usually present no problem with low or high pH. Of course, in arid areas with alkali soils, pond waters may naturally have a pH higher than ideal for fish culture. In some situations, good sportfish ponds cannot be established in arid areas.
- Low pH can usually be remediated satisfactorily through the application of agricultural limestone.
- In some areas, ponds naturally have low total hardness (calcium and magnesium result in hardness) and rather high alkalinity. Such ponds may develop very high afternoon pH, even up to 11. Gypsum (calcium sulfate) treatment increases calcium concentration and, as a result, tends to lower the pH.
Dr. Claude Boyd is a professor emeritus in the School of Fisheries, Aquaculture and Aquatic Sciences, Auburn University, Auburn, Alabama 36849. His work with water quality is internationally renowned. His most recent book, Handbook for Aquaculture Water Quality, is a must for anyone interested in learning about water chemistry and how it relates to your pond. It's technical and thorough but easy to read and understand. Buy it at www.pondboss.com in the online store.
Reprinted with permission from Pond Boss Magazine
