I have a friend who grows in 6-inch pots something of a ScrOg and he puts 50 plants/light. he gives em a week of veg and flips them right away. By the time they are done, they are not more than 2 feet tall. never had rootbound issues.
Or second solution and i can't say this without tearing...
@DemonTrich
20k ppm that's more than lethal for plants. Poor things.
Ok so your plants don't get any noticible shock from you bumping it up to 1000ppm right away?
Did you try your same setup without co2?
Was there a big difference?