Whats driving the PH?

[fio1022]

I'm planning to add my first calcium reactor soon.Limewater just can't keep up anymore.
I understand how they work but I have a basic question that I'm sure you chemistry wizards will have no problem with.
You inject co2 into a calcium rich media which is at a PH of about 6.5.When this effluent returns to the tank why doesn't that lower the tanks ph?What is driving/maintaining the tanks high ph with an effluent so low?
Please go easy on me.Thanks:)

[bertoni]

The reactor will tend to lower the pH of the tank a bit, but the carbon dioxide will outgas from the system, so the pH often doesn't drop much, if at all, if the system has plenty of aeration. Some people run the output of the reactor into the skimmer for this reason.