Pretty much in agreement to the above and just want to add more food for thought:
Object is to inject enough CO2 to supplement removal of "naturally occurring" CO2 levels.
Judging CO2 addition by the shotgun method of keeping CO2 at say 30ppm is equiv to dumping a bag of fertilizer in to keep the levels high.
Ideally one would add CO2 in a manner that just replaces a certain level of CO2 that the plants can use w/ little daily excess..
Obviously it is easy to say but not easy to do.
Things like poor mixing (shouldn't apply that much really), natural outgassing to atmosphere (probably relevent) and then ACTUAL utilization levels and then hitting other limiting factors (light, fertilizer) make it quite difficult
to define what is an "ideal" level of CO2..
Freshwater Planted Aquarium Care and Maintenance: CO2 in the Planted Aquarium
CO2 Basics for Aquarium Plants
An ideal CO2 level is 15 to 30 ppm. However, to allow for inaccuracies in measurements, you should target about 20 to 25 ppm. If you get a lot higher than 30 ppm, your fish will be stressed, and they could die if the CO2 is around 50 or more. It's hard to get lethal levels of CO2 into the aquarium, especially with a fermentation container. Lower than about 15 or so ppm, you will not see much effect on your plants.
I'd like to see the studies that determined this. I have no doubt that added CO2 can be a benefit.. I have doubts as to the effective level, and certainly it will move depending on other circumstances.
and to add to the mess. some do not see the benefit at all..
Debunked: CO2-Myth --- See tanks without carbon dioxide fertilization | MonsterFishKeepers.com
OK found some real data.. please see chart pg5..
Between 1400 and 5400 LUX and increasing CO2 concentration from 6.6PPM to 35.2PPM you only get an increase of daily growth of say.. 0.3-1%
HOW they extrapolated that to say "25-50 mg/L will improve plant growth". is beyond me. Maybe someone else can explain that one..
Even at 15200LUX and increasing CO2 from 6.6 to 35.2PMM only increases daily growth 4.3%..
Besides trimming every week is a PIA..