It does both. when water exits the first chamber the CO2 level is still quite high. Usually still around media melting levels (depends on what you have it set for) The second stage will use that CO2 to convert more media. This causes the PH to rise. So at some point the CO2 levels are no longer effectivly lowering PH to the media melting point as it passes through the second stage.
So the main function of the second stage is to bring up the PH prior to the water going back into the tank, but it will melt some media.
Now you can adjust your CO2 to melt more media in the second stage.
J
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