Please ignore the recommended ranges for good/Bad CO2 ppm, they vary by tank and lighting and other factors, a good range to target initially is 30-40 ppm. Then adjust slowly watching plant health and vigor.
Measured your KH, then see how much you need to reduce the pH to get your target CO2 ppm.
Say you tap water is a KH of 5m say you want 35-40ppm of CO2, you should add enough to get the pH to 6.6 and be able to keep it there.
Warning, KH may not be entirely carbonate hardness. This means you will think and believe you have MORE than you actually do, thus you may be under dosing CO2.
This issues will never be the reverse, eg, you are adding more CO2 than you think.
So the error is always on the safe side usign this method.
As the KH in your tap drops, say your KH is 1-2 degrees, there's just not much room for other sources of KH other than carbonate, at 4-5 and above, there may be.
So assuming most of the KH is carbonate hardness for a KH or 1 degree is likely okay.
Say you want a CO2 of 50 ppm for a KH of 1 degree? the chart does not cover those ranges of pH's, but you can scale using a similar higher KH to see what the pH adjustment would be.
So about 5.9 pH would give about 39 ppm and a pH of 5.8 would give about 48 ppm of CO2.
I typically use a pH meter, American marine works pretty good.
Then a nice CO2 reg, check valve, needle valve etc to dial it in.
I would say 1-2 bars into the red is ideal also in the above graph.
Another good item to use for KH: Hanna alkalinity meter is pretty good IME, same with the Lamotte alkalinity test kit.
Test tank water and tap water every month or two as a good measure. Over a season, KH can change from some Tap supplies.
Mine goes from 1 to about 2 each season. About 19-20ppm to about 35-38ppm.
So I'm in the 5.8-5.9's to 6 flat for pH's mostly.
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Measure your KH, then see how much you need to reduce the pH to get your target CO2 ppm.