A modicum of progress.
I put together the manifold I'll be using for the inline pH probe and the co2 injection.
Water flows from left to right, first hitting the pH probe mounting using the handy dandy Heyco liquide tight cordgrip, and then onto the co2 injection. I'm using an irrigation drip connection. Great little bit of plumbing. If you've never used one, it's perfect for this. You slide in your 1/4" line, then give it just a little tug, and it seals up around it like a chinese finger trap.
Had to make do with a couple of fittings from the local Ace Hardware(so much quicker than driving out of the way to Home Depot) so I'm gonna have to let the PVC glue seal up.