by John Tullock
Carbon dioxide is essential to plants, but a carbon dioxide system is not essential for some planted aquariums. Tanks with slow-growing, low light plants such as ferns and many species of Cryptocoryne do just fine without supplemental carbon dioxide. On the other hand, a large, brightly illuminated tank filled with fast-growing stem plants will soon begin to look straggly without supplemental carbon dioxide.
I explained in a previous post that the carbon dioxide concentration can be estimated when both the pH and KH are known. If the KH is in German degrees (dKH), then the relationship is ppm CO2 = 3KH * 10^^(7-pH). That second term is 10 raised to the power of 7 minus the pH. Thus, when the pH is 7, this term equals 1, and the equation reduces to ppmCO2 = 3KH. This simplification assumes that only carbonate and bicarbonate ions are responsible for the alkalinity, an unlikely scenario in real life. Nevertheless, the estimate is good enough for horticultural purposes. Most experienced aquarists recommend about 15 ppm of carbon dioxide.
Equally experienced hobbyists dispense with measurements altogether and go by bubble count. The rate of introduction of CO2 is adjusted based on the appearance of the plants. Many posted recommendations hover around 2 bubbles per minute as a good starting point.
Always remember that too much carbon dioxide can be harmful to fish or invertebrates. If you are installing a system on an existing tank, watch carefully for signs of stress in the animals. Dial back the CO2 if they seem stressed-out or behave in an unusual manner.
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