How to set the target level of co2 in a planted tank
There is a relation between pH, KH and CO2. When we add CO2 to water, it forms carbonic acid, which lowers the pH.
The more CO2 that gets dissolved into the water, the lower the pH.
Working against the CO2 to raise the pH is the KH (the Carbonate Hardness). Assuming a constant amount of CO2, a higher KH, will result in a higher pH.
Over the years, aquarists have found ways to use this relation to calculate the amount of CO2 dissolved in water.
Unfortunately, it is known that other buffers such as pH-UP, pH-Down, Discus Buffers and Phosphate will throw off this relation making most test kit and calculation methods inaccurate. Most buffers can be avoided but unfortunately, there will/should always be a certain amount of phosphate in our planted tank.
A well known technique is to use accurate pH and KH test kits and to read off the result on a CO2 table.
It is cheap and easy to do but unfortunately, other buffers present in the water makes it inaccurate.
If you chose to use this method, remember to avoid using buffers and that phosphates will have an effect on your results.
You have KH = 4, then your pH should be from 6.6 to 6.9 (green area 32-16 ppm CO2).