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Ok, explain me why I have never been able to read the KH/PH chart: I have KH 7 and the PH from tap is 7.4. According to the chart, I'd need to lower my PH to just 6.9 to approach 30ppm of co2... Correct?

But the truth is, with my 75gl tank, EI and a big wet/dry filter, I need to lower the PH to 6.1 to see my plants grow enough well with very little algae. Fish are good, and yet the chart would give me a crazy 166ppm co2 concentration!

That cannot be the case, right? Please, note I don't use any PH buffer nor a tap water softener... And my tap water doesn't contain phosphates...

So, what's wrong?
 

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Thank you for the responses! All responses seem legit. Would the following conclusion be correct....the ph swings are real,but at long as all the 'stuff' (aside from co2) in the water is stable my fish will be Ok?? Please correct if I am completely wrong.
Yes, that's correct.
 

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Phosphates aren't the only non-CO2 thing in water that affects the pH. We have organic acids from wood or decaying leaves that also lower the pH. The relationship between carbonate hardness, CO2 concentration and pH is only applicable for pure, distilled water with nothing else dissolved in it except a carbonate compound and CO2. Add some tannic acid and the relationship breaks down. If this were not the case you could gain CO2 by adding hydrochloric acid to the water, thus lowering the pH. But, you only gain CO2 in the water by dissolving CO2 in the water.
Well, I appreciate your answer, but doesn't explain my question... I don't think to be the only one experiencing this mismatch. For me the chart is completely useless.
 

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This is a very nice answer, and pretty much my own thinking! Makes perfectly sense, thank you!

So, I guess, if you use tap water, you have just one option: experiment until you find the right amount of co2 your plants need, and the so called "sweet spot". So hard to find for me!
 
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