I actually did this some years ago. I started out with an arduino, but ended up doing this with x-10 modules and a set-point ph monitor. The monitor sent a signal to the x-10 wall module, which was sent over the home wiring to a usb wall outlet receiver and finally to the computer. I used a wall x-10 wall receptacle to plug a surplus medical dosing pump into to deliver the carbonate. The whole thing was programmed and controlled by (I forget the name) x-10 software. I got the idea and instructions from someone else on ReefCentral.com. The discussion is probably archived.
Mike