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You see a drop in ph because co2 forms carbonic acid with water, your actual ph never changes. If co2 actually changed the physical make up of water (GH and KH) then there would be no use for RO water systems. Co2 does not remove GH or KH, these are the factors that alter the real ph of your water.
This ph drop is a false ph drop. Think of it as co2 tricking your ph test kits or ph controller. Test kits and controllers are simply responding to contact of the co2 acid, the physical nature of the the water never changes, always remains the same. You can inject co2 until the ph controller reads 5.0 if you wanted to, the real ph is still the same. We know this because the minute you take a sample of the water in a cup, shake it up to gas off the co2 the ph go's right back up to where you started. Never once did you remove GH or KH from the water, the only real way to ever alter the ph is to use RO water, or rain water. These types of water contain no GH or KH, no minerals.
I don't agree. The pH change is real. When anything in the water changes the acidity of the water the pH is a measure of that. Adding an acid, or something that is converted to an acid to any degree, as CO2 is, changes the surplus of H+ ions in the water, which is what a change in pH is. When you take a sample of the water in the tank and shake it up, you are removing the CO2, which also causes the amount of carbonic acid in the water to drop, reducing the surplus of H+ ions, as indicated by the rise in pH. Aquarium water pH is a result of several things in the water, it is a consequence of "stuff" being in the water. What harms fish is some of that "stuff", not the pH itself. (This is advanced chemistry explained by someone with only bare knowledge of any chemistry.):icon_cool
 

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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?
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.
 

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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.
The chart is useless for determining how much CO2 is in the aquarium water. It does have value for when you are working with distilled water and baking soda, doing experiments. Measuring how much CO2 is in the aquarium water is very difficult unless you have a few thousand dollars to spare for a quality CO2 measuring probe. You can assume you have a reasonably high CO2 concentration if you get a 1 pH drop when you turn on your CO2 after allowing lots of time for the CO2 that was in the tank to dissipate. It will probably be around 30 ppm, but it could be much lower or higher.

It is better to judge whether you have an optimum amount of CO2 in the water by observing the plants and fish, and making very small changes in CO2 bubble rate until both the plants and the fish are doing well.
 

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same here. my Kh>7
with CO2 PH drops to <6.6 and reading chart that gives me loads of co2 wich cant be true using diy yeast system
What is the starting pH when it drops to 6.6? Remember, the drop in pH with CO2 addition does tell you the ratio of the final ppm of CO2 to the initial ppm of CO2.
 
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