Degas tank water by letting sit in a bowl for about 24 hours. Compare that PH to the PH of the tank with co2 running. A 1 point drop is approximately 30 ppm. That is the most reliable way outside of an expensive test. Forget the KH/PH charts, as already mentioned too many variables affect the KH.
But really what good is knowing whether you have 29 or 36? What can you do with that number that cant be done with the PH drop? If a 1 point drop is too much for the fish, try .8 or .9. If a 1 point drop isnt enough for plants (you'll just have to watch them for a few days) try 1.1 or 1.2 etc.
You'd be doing the same thing if you knew the exact ppm, 31 ppm too much, go down to 27. 31 ppm not enough, go up to 35-40
I understand wanting to know specifically but that knowledge, in my opinion, is no more useful than using the relative PH drop. All you are getting from either one is a reference point from which to raise or lower according to what is needed. And to know what is needed for your particular set up you will have to watch the livestock and also the plants regardless of which metric you go by
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