NOTE: If you aren't adding CO2 to your water, and the CO2 level based on the pH and KH indicates more than 5ppm, then it is very likely that some other buffer (such as phosphate) is present in your water. In an inhabited aquarium, the amount of CO2 produced by the fish will not have an effect on CO2 levels in the water. Any excess CO2 created by fish will dissipate into the air, leaving a fairly constant CO2 level of about 3-4ppm. If you test your pH and KH, and without adding any CO2, the chart says you've got 20ppm CO2, don't believe it.
In some case, water coming right from the tap can contain very high or very low levels of CO2. This can result in tap water with a high KH, and low pH. But, in just a few hours, that excess CO2 will dissipate from the water, leaving the normal 3-4ppm, and the pH will rise. Sometimes, the water might come from the tap with extremely little CO2, which can result in tap water with a low KH, and a very high pH. Again, after a few hours, the CO2 level will equalize, and the water will end up with 3-4ppm CO2.
