pH is the measure of active free floating Hydrogen ions in water. A pH level of 7 is neutral, a pH level below 7 is acidic and a pH level above 7 is alkaline. The method used to calculate pH is called the Michalis-Menten equation. The more free floating Hydrogen Ions you have in your water, the lower your pH will be. The more free floating Hydroxide Ions you have in your water, the higher your pH will be. Koi can live comfortably in water with a pH range of 6.5 - 8.0. If a pond has a low pH level, which is often caused by the Nitrogen cycle or aquatic plants (aquatic plants put out Carbon Dioxide at night, which changes into carbonic acid H2CO3), it can be raised by adding calcium carbonate (CaCO3) to the pond. This will raise the pH level and will also act as an acid buffer. Calcium carbonate will keep your pH at a certain level, for it has the ability to hold or release free Hydrogen ions.
If a pond has a high pH, which is caused by large amounts of carbonates, such as exposed concrete, limestone or hard water, it can be lowered by injecting Carbon Dioxide into the water. As mentioned above, Carbon Dioxide changes to carbonic acid (H2CO3) and then lowers the pH. Acid can be used too, but it tends to remove any buffer fast and will cause a pH crash. When the pH of a pond crashes, the water turns acidic and loses its capability to harbor life. pH can contribute to the toxicity of Ammonia too, if the pH in a pond is lower than 7.0, it will transform the toxic Ammonia (NH3) into harmless ionized Ammonia (NH4).
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