| CO2 Measurement
Here is a handy little chart for determining CO2 levels in a planted tank. It is far more accurate than the KH pH chart commonly seen.
Change in pH PPM CO2
0.5_____________ 13
0.6_____________ 16
0.7_____________ 20
0.8_____________ 25
0.9_____________ 32
1.0_____________ 40
1.1_____________ 51
1.2_____________ 64
1.3_____________ 80
1.4____________ 100
Take a sample of tank water and let it degas and come to equilibrium. Overnight is the time I use.
This chart assumes that the normal equilibrium of CO2 and a small body of water will be approximately 4-5 ppm.
Injecting CO2 drives the pH down.
Test the pH of the sample water and then test the pH of the tank in which CO2 is being dissolved. The difference will give you a dissolved CO2 quantity in parts per million. | 
