So now I had this idea. Add a CO 2 air-valve after the tank regulator and follow that with the needle valve. A control box I would design and build would actuate the air-valve at the proper pH setpoints by sensing the tank pH via a pH probe. The next problem was to decide how to mix the CO 2 bubbles efficiently into the tank. In commercial systems, the thing that does this is called a reactor and is composed of a long tube with things like bio-balls inside.
Water is pumped into the top and comes out the bottom.
CO 2 is bubbled into the bottom and mixes in the column with the water. What I did was get a plastic T-valve at an aquarium store and cut the inlet into my 100g tank's Magnum 220 filter. I put the CO 2 gas into the one section of the T and it mixed in with the incoming tank water inside the Magnum. With experimentation of the needle valve flow rate, you can get this to happen quite easily. In fact, for most people, just that part mentioned so far would be enough to get a practical CO 2 system running.