Questions about Ca reactors

[BCOrchidGuy]

I've yet to find a drawing of a Ca reactor that explains what the parts do, I would like to build one but I don't understand the system its self. I know that it takes tank water and mixes it with CO2 and forces that acidic water through the carbonate material but, is that it? does it just loop it through?

I was thinking about making something along the lines of a Ca reactor but having it run through my freshwater auto top off, just have a bucket with some aragonite and a bit of CO2 bubbling away in there... I figure if it went into my sump then through my pumps etc and then through the water fall back into my tank that should rid the water of excess CO2.... shouldn't it?