Get a drop checker, it's the only way to be sure about CO2 ppm.
Forget about your tap pH in relation to your tank's pH. The tap contains things that effect pH, which will precipitate out after a day or so in an open container, the pH it settles to is closer to the actual pH of your tap. After being added to the tank, even without CO2 injection, pH often settles even more due to a carbonic acids built up in the tank, add CO2 and it settles more. For instance, my tap is pH8.2, after 24-48hrs in an open container, it settles at 7.2, added to the tank without CO2 injection it settles to <7, turn on CO2 and it settles to 6.4~6.6. Once you understand this process in your own household, then the pH/KH chart can be useful, or else a drop checker is the only way to go.
Also, if you're one to worry about this, it is more important to make sure the source water going into the tank has the same KH and GH levels and temp of the tank rather than the same pH, due to the precipitating compounds in the tap. People not understanding these properties is what the makers of snake oils like PH-Up and PH-Down are counting on to be able to steal your money and trick you into creating a bigger problem in your tank, fooling you into buying more of their product to try and fix the "problem". Ph can constantly change from properties that the fish are not concerned with, they want consistent KH, GH and temp. The pH drop from CO2 only concerns them if CO2 levels are too high, then it's the CO2 ppm that is the problem, not the pH. The long term pH drop from a buildup of carbonic acids only effects wild caught hard water fish, which are not suited for acidic water, the issue here is the acidity of the water. PH can be a guide to figuring out other important parameters but is not the sole parameter that need s to be adjusted, if it is too low or too high, figure out what is causing it and address that rather than concentrating on the pH.
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