I like the idea. I originally thought you had the PH's injecting the CO2
at/near the surface and thought that might not be a great idea. But now I see you are using the PH's more like an air pump in an UGF
to push the CO2
to the bottom of the tank and out the spray bar along the substrate.
My only concern with the idea is the forcing of the bubbles down the tubing. For your size tank there would (I imagine) be a good bit of backpressure, not to mention the gas will attempt to go back up the tubing. I would use very small diameter tubing for the downpipes to keep the velocity of water/gas as high as possible so you won't get huge bubbles forming in the tubing that might disrupt flow. The other part to that concern is what happens when the CO2
gets to the spray bar. I have an inkling that you will get 1 or 2 streams of bubbles along the spray bar instead of an even dispersion across the whole bar.
After my short time with CO2
injection I feel the best way to get good saturation is to have injection into a powerhead at/near substrate level. This gives you a fine mist that will get dispersed around the tank and slowly float upwards all the while dissolving in the water. The only problem, as you mentioned, is to limit turbulence on the plants surrounding the PH. I have a large terra cotta pot in my 20 gallon that effectively disrupts the flow of the PH, as well has having the PH set on its lowest setting. This results in an almost imperceptible current at substrate level. MUCH less than when I had the PH near the surface shooting downwards, and I get better saturation since the bubbles now have to travel a foot upwards instead of a couple of inches. If you have a piece of drift wood, other ornament, or stiff stem plant such as anubias, you can fire directly into this, and it should have a similar effect.
After trying a TON of variations on a lot of different methods, I've always come back to the simplest tends to be the best, that is force the highest contact time by injecting at substrate level, and letting the slow ascent of the microbubbles do the work for you. This also results in the least amount of "stuff" (piping, "reactors", etc) being needed in the tank. For larger tanks like yours this probably isn't much of a concern, but for those of us with small tanks (mine's a 20gallon high), I need to limit the mechanical clutter as much as possible, otherwise the fish would have no where to swim!
Let us know how it goes if you decide on your method because it is unique!