Many of us think that the low pH of the tank water is caused by excess CO2 in the effluent that has not sufficient time to react and dissolve the media.
Actually the low pH of the tank water is caused by the fact that the CR is adding alkalinity mainly in the form of bicarbonate. The bicarbonate itself will cause the pH of the tank to drop. When the excess CO2 is degassed (eg dripping effluent into protein skimmer etc) then some of the bicarbonate will be converted into carbonate. In case of this, once the water becomes supersaturated with carbonate then the carbonate will be inclined to precipitate onto calcium carbonate surfaces, and some alkalinity will be lost.