KH PH CO2

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KH (Carbonate Hardness - Karbonatharte in German)
Carbonate Hardness measures the amount of carbonates and bicarbonates in water, expressed in German degrees of hardness (dKH). The term 'hardness' in KH is somewhat confusing because it does not actually measure hardness, but rather the alkalinity (buffering capacity - ability to neutralize acids) of a solution to resist a pH change. The higher the KH, the more stable and resistant your water is to pH swings. A KH of 2-3dKH is generally accepted as the minimum to maintain a stable pH.

pH (Per Hydrogen)
pH is the measure of the balance of Hydrogen (H+) and Hydroxide (OH) ions in water. The pH scale goes from 0-14. As most of us know, a pH reading of 7.0 means that the water is neutral, with readings from 0-6.9 as being Acidic, and from 7.1-14 being Basic. pH is also a function of KH and CO2 concentration. That is, we can determine the amount of CO2 (mg/L or ppm) in the water if we know the pH and KH values. In aquarium applications, the pH will fluctuate when we add CO2, while KH remains stable.

Carbon Dioxide (CO2)
Carbon dioxide is not named in the title of this article, but it, too, has an integral part of water chemistry. CO2 exists in water in far great concentration than either oxygen and nitrogen combined (70:2:1). We often think of injecting CO2 to increase the growth rate of plants in our tanks. However, its direct relationship with KH and pH should be understood as well. Without going too much into chemical processes, when carbon dioxide is dissolved in water, it becomes carbonic acid, increasing the amount of acids, and lowers the pH in an aquarium. The amount of carbonates (KH) present in the water will determine how far the pH will drop. Carbonates present in the water will buffer (neutralize) the carbonic acid released when CO2 is injected, therefore, buffering the pH