Just to brainstorm a bit: If I had a plants only tank, and wanted to maximize the plant growth rate, I think I would use pearling as my guide. I would raise the bubble rate enough to get good pearling of all plants. Of course I would also dose per the EI method, and make sure I had high light - maybe 60-80 micromols of PAR. And, I would probably try a wet/dry filter to keep as much oxygen in the water as I could. Adding more CO2 wouldn't hurt anything that I can think of. And, running the CO2 24 hours a day every day would waste CO2, but not hurt anything, while it would help discourage algae from starting.