One of the secrets to lush aquarium plant growth is the use of carbon dioxide CO2 fertilization. Just how much CO2 is enough, and how do we measure it?
Reliable CO2 test kits tend to be rather expensive, and are used by water quality experts. You can find them from companies like Hach and LaMotte. Tetra produce a hobbyist grade kit, but it is not readily available.
On the other hand, CO2 concentrations can be calculated measuring the carbonate hardness/alkalinity, or KH and the pH of the water. Both of these test kits cost just a few dollars, and give us a 'close enough' measurement of CO2 in planted tanks by using the following formula:
CO2 (in PPM) = 3xKH x10(7-pH)
where KH is KH in degrees.
(Formula from Chuck Gadd)
If your test kit for KH is in ppm or mg/L, you can convert to dKH by dividing the test result by 17.9 (i.e. 1 dKH=17.9 ppm).
Please note that the calculations assume that no other "acid buffers," pH reducing chemicals or peat filtration is being used in the system. These substances invalidate the calculations.
The expression can be shown in a chart, as below. Concentrations of approximately 15-25 ppm of CO2 are generally considered good for plant growth and safe for fish. Values above 25 ppm CO2 may cause some difficulties with fish, but many have reported no ill effects to their fish at levels of 30 and 35 ppm. If you want to increase your CO2, watch your fish carefully. If you see them gasping at the surface in the morning before you lights come on, dial the CO2 back.