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RickF
02-07-2014, 10:19 AM
We often see questions along the lines of “what is the ideal pH for koi”, “ how do I change the pH of my pond” or “the pH of my source water is too low (or too high)”. Before I answer these questions, let me start off by explaining what pH is and what controls what the pH will be.

Water, as you probably already know, is H2O – two hydrogen atoms bound with one oxygen atom. In solution, some of the water breaks apart into one hydrogen ion (H+) and one hydroxide ion (HO-). pH is a term that quantifies the relationship between the concentration of free hydrogen ions and the concentration of free hydroxide ions. Theoretically, pH can range from a value of 0 to a value of 14; however in the real world, a value below 1 or above 13 is nearly impossible to attain.

The lower the value of pH, the higher the concentration of hydrogen ions compared to hydroxide ions, and each integral value represents 1/10 the concentration of hydrogen ions and 10 times the concentration of hydroxide ions compared to the lower value; thus a pH of 8 means that there is 1/100 the concentration of hydrogen ions and 100 times the concentration of hydroxide ions compared to a pH of 6.

In pure, distilled water, the concentration of hydrogen ions is equal to the concentrations of hydroxide ions, and the pH value is 7.0. When other compounds are dissolved in the water, the ratio of hydrogen ions to hydroxide ions can change, which is reflected in a change in pH. Acids are compounds that add more hydrogen ions to the solution than they do hydroxide ions, and thus, lower the pH. Bases are compounds that add more hydroxide ions to the water than they do hydrogen ions, and thus, raise the pH. Salts are compounds that have no effect on the pH. The water in the pond is never pure water. In fact, fish do not fare well in pure water. There are always other things dissolved in the water, and many of these things affect the pH.

So adjusting the pH of the pond or source water sounds simple. If I want to raise the pH, I can add a base, like sodium hydroxide, and if I want to lower the pH, I can add an acid like hydrochloric acid. In reality, it is not that simple. If a salt and its weak acid are both present in the water (for example carbonic acid and sodium bicarbonate) a situation exists where hydrogen or hydroxide ions can be either accepted or donated to maintain the pH at a constant value. This is known as a buffer and this ability of the water to resist a change in pH is quantified by a value known as carbonate hardness (KH). See the post “KH for Laymen and Newbies” (http://www.koiphen.com/forums/showthread.php?143231-KH-for-the-Layman-and-Newbie) for more about KH. The actual pH depends on what the buffer is, but if the KH is high, the pH, regardless of what it is, will be difficult to change.

If the source water or pond has a high KH, and you add an acid to lower the pH, what will happen is that several small additions of the acid will have no effect on the pH, then at some point, the next addition will cause a drastic drop in pH, and a drastic drop in pH can be fatal to your fish. What is happening is that additions of the acid cause a drop in KH, but no change in pH until the point is reached where the KH is no longer sufficient to maintain the pH. When this point is reached, the next addition of acid will cause the pH to plummet.

Before you consider doing anything to change the pH of the pond or source water, first measure the KH. If the KH of the source water high, there is no point in trying to change the pH. Koi are extremely adaptable and will do well at pH values between 6.5 and 9 as long as the pH is stable. A high KH ensures a stable pH, so just accept that the pH is what it is, and unless you want to go to the expense of using a reverse osmosis filter to lower the KH of the source water, attempts at changing the pH will be either futile or dangerous for your fish.

If the KH or the source water is low and the KH of the pond is high (presumably because you have been adding things, e.g., baking soda, to the water to raise the KH), then the pH of the source water is irrelevant. The source water is not going to change the pH of the pond (although it will lower the KH). If the KH of the source water is low and the pH is low, then the low pH might be caused by an excess of carbon dioxide in the water. This is especially likely if the source water comes from a well, but it can also happen with municipal water. When carbon dioxide is dissolved in water, it creates carbonic acid, which lowers the pH. Aerating the water before or as it is being added to the pond will raise the pH of the source water and protect your fish from potentially lethal concentrations of carbon dioxide.

If the KH of the pond is low, then there is a great risk that the pH will change easily. Rapid change in pH is dangerous to the fish. In a pond with one koi per 250 gallons and without a bead filter or other filter that has the potential for low oxygen concentrations, the KH should not drop below 80 ppm. If there is a bead filter or other enclosed filter, the KH should not drop below 150 ppm. Higher stocking densities (although not recommended) demand higher KH to ensure that there will not be a pH crash. pH is maintained by maintaining the KH, and there should be no attempt to change the pH directly.