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I like your site, very nice. I liked the CO2 calculator by the shake method, except after some experimentation I found that there is a flaw in the formula (I think, I'm new so I may be wrong).

Using the numbers from my tank
KH is 5 dH
pH is 6.8 before shaking
pH is 7.9 after shaking
calculator tells me my CO2 is 6 ppm by the shake method

now if I put the numbers in the KH/pH calculator
5 kh & 6.8 I get 24 ppm
5 kh & 7.9 I get 2 ppm

Since the after shaken result of 2 ppm makes sense considering equalization with the atmosphere I would imagine that the 24 ppm is reasonably correct too. So there must be something wrong with the calculation that gave me 6 ppm.
 

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I did a little algebra and came up with this equation. I assumed that the CO2 level after shaking is equal to 2. The 2 can always be changed to whatever is determined to be the equilibrium level of CO2. This result checks with the numbers I had in my previous post.

Let a = pH before shaking
Let b = pH after shaking

CO2 = 2 x 10^7-b / 10^7-a
 
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