You know I don't think anyone has yet to invent a *great* system to get CO2 dissolved. You could even add another whole house filter with the scrubbies and downpipe that would give additional dwell time to the CO2 that way. There really is no hard fast rule. I wish I could recall who it was that was experimenting with a recirculating system that recycled the water through a CO2 chamber more than once. It got some interest but then it sort-of fell off the radar and I really wonder what happened to that. The system looked a lot more complex than the stuff we're playing with here. It was pretty slick (practical? I dunno). Suffice it to say that each aquarium is different and each system has to be designed and tweaked to that particular tank. Sometimes it's seems easier to cram a boilerplate concept into an answer to a challenge and forget that the overall goal is a healthy ecosystem that might require more creativity and of course, money
Have you ever checked out the carbonator from a soda fountain? LOL - I have!