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So I have always used the the KH/PH/Co2 measuring method, as well as a drop checker. Always been accurate to me. But someone on another thread says that the KH/PH/Co2 relationship, doesn't work. Can someone explain this to me or point me in the direction for info on this topic, i'm very curious now. Thank You!
 

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Here's a good place to start.

Especially the section "The pH-KH-CO2 Relationship".
 

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In short, the kH/pH/CO2 relationship only works if carbonates are the only buffer present in the water. Unfortunately, in the aquarium, we often add phosphates for the benefit of plants, and the presence of these buffers will throw off the relationship.

As a result, using a drop checker with a 4 dkH reference solution (whereby the only buffering agent present is carbonate) is more accurate than using the kH/pH/CO2 relationship when measuring the kH and pH of the aquarium water (as opposed to the water in a drop checker).
 

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If the CO2 you add drops the pH by 1 full point in your tank then your getting approximately 30 ppm of CO2. If other organic acids in your tanks are causing your pH to drift over time, then it could introduce errors into your calculated CO2. You can double check this by taking a sample of tank water, allowing the CO2 to offgass and retesting pH, confirming the 1 point drop from equilibrium.
 
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