Originally Posted by Tom Barr
Co2, all you have to do is check the pH/KH chart for CO2.
I was a bit confused as to your user name at first, but that's another story.
In any case, you cannot measure pH and kH to get CO2 reliably, as there are other factors in aquarium water that will affect the kH.
The best way to check CO2 levels is to use a drop checker with a 4 dkH reference solution.