I believe that the only way to change the amount of
CO2 is to add
CO2 a change in pH will not make it any higher. I'm sure someone will know for sure though.
Edit: This was at Chucks planted aquarium pages
Myth:
CO2 level can be adjusted simply by adding chemicals to alter the
KH or pH.
This is a common misconception when using the
CO2 /
KH / pH table. It appears that by altering any parameter, the other values should move. But this is not true. Treat the pH value you see as a result. If you alter the
KH, then the pH will move. If you alter the
CO2 level, then the pH will move. The pH will always react to changes in either of the other two parameters.
Example: My water comes out of the tap with a
KH of 3 degrees, and a pH of 7.6, which according to the the indicates a
CO2 level of 2.3ppm. Looking at the chart, I might (incorrectly) assume that If I simply raised my
KH to 10 degrees, I would end up with the same pH, but the
CO2 level would now be 12ppm! How easy! I can add
CO2 just by adding some baking soda to raise my
KH.
BUT! it doesn't work that way. Instead, as I raise the
KH, the pH will rise along with it, and the indicated
CO2 level staying at it's 2.3ppm. In my case, if I raised the
KH to 10 degrees,
You can not alter the
KH levels other than by adding or removing carbonate. You can not alter the
CO2 levels other than by adding or removing
CO2.
Adding certain "pH altering additives" can cause much confusion as well. Additives like "Proper pH 7.0" which force the pH to a certain value completely invalidate the
CO2 /
KH / pH relationship. This is because these pH altering additives contain phosphates. Phosphates replace the carbonates in the buffering system. And the
CO2 /
KH / pH relationship is only valid in a system that is buffered by Carbonates.
There is on case I've seen where the addition of
CO2 resulted in an increase in
KH. This can happen when you have something in the tank that dissolves carbonate into the water. Seashells, crushed coral, and many gravels and rocks will do this. With the addition of
CO2, the water turns more acidic, which will increase the dissolving of the minerals. It appears that increasing
CO2 raises the
KH, which isn't really the case. The dissolving minerals raise the
KH, and the increase in
KH results in an increase in pH. In a system using a pH probe and controller to regulate
CO2 levels, this can have fatal consequences, since the pH controller will keep trying to lower the pH, but as more
CO2 is dissolved, it lowers the pH, which raises the
KH, which raises the pH. So you now have more
CO2, but the same pH. So the controller adds even MORE co2. And it will keep going. So it's important to know your
KH whenever using pH to judge
CO2 levels.
