How to Raise the Low Alkalinity of a Pond

Keeping a fish pond on your property requires careful attention to the pH of the water. The pH is a measure of the acidity or alkalinity of the water, which can affect the health of your fish. The pH scale ranges from 0 to 14, with numbers under 7 indicating high acidity, numbers over 7 indicating high alkalinity and a reading of 7 indicating a neutral condition. Alkalinity can also refer to the ability of the water to deal with acidic waster products from fish and plants. When alkalinity of the pond is too low, you must raise the pH to prevent the acidic water from damaging the fish’s skin.


Test the water at several times of day with a pH test kit. Keep in mind that the pH of pond water naturally lowers and rises throughout the day and night because of changes in levels of carbon dioxide caused by photosynthesis. The pH lowers at night because plants in the water do not photosynthesize when there is no sunlight. Similarly, the pH rises in the afternoon after the plants have photosynthesized for a period of time. Always consider the daily natural fluctuations of pond chemistry before attempting to adjust the pH. Optimum pH for ponds is between 7.2 and 7.8 on the pH scale.


Add a chemical buffer to the water to stabilize the pH. Hard water has a high buffering capacity, which is the ability of the minerals in the water to prevent extreme pH changes. Buffers in hard water acts like a sponge to soak up and neutralize chemicals in the water that can change pH. Soft water has little ability to stabilize these chemicals, so pH can change quickly and dramatically. Buffers add acid and alkaline salts, such as clorides and hydroxides, that convert to carbonates and bicarbonates in the water and provide greater buffering capacity to minimize fluctuations.


Mix in a compound that raises the pH value, such as calcium carbonate, limestone or eggshells. Pond supply stores carry a number of pH-raising products for this purpose. Add the pH-raising compound in small amounts, to keep from making the water too alkaline, which will create an additional problem.