Years ago Ole Pedersen, et.al, wrote a scientific paper about Liebigs Law of minimums, related to light and CO2. Later the paper was simplified and was available on Tropica's website, as http://www.bio-web.dk/ole_pedersen/p..._2001_2_22.pdf
In that article was a table of data showing the effect of light and CO2 on Riccia growth rates. That table was:
After studying that data for awhile I decided to plot it on a graph in a different way to see what I could learn. That gave me this:
This shows me that, for Riccia, it takes very little CO2 to get the full benefit of CO2 at light intensities up to at least 90 PAR, very high light. One reason for this is that Riccia is a low light, slow growing plant. It is reasonable to assume that other low light, slow growing plants would have given similar results.
Even high light plants, or fast growing plants probably give similar results, but with somewhat more CO2 needed to get the maximum growth rate. I grow Hygro corymbosa siamensis in my 30-35 PAR tank, and I see a similar result as that shown for Riccia.
I started growing the Hygro with no CO2, just Metricide as a carbon source. For over a month the Hygro struggled to grow, getting only a little bigger in a month. So, I added DIY CO2 at around 1 bubble per second. The Hygro immediately started growing much faster, so much so that I have been pruning about 4-6 inches off each plant every week.
To check how much I had affected the amount of CO2 in the water I put 0.5 dKH water in a drop checker and used it to monitor the CO2. With that low KH water, the range of measurement for CO2 is about 1 ppm to about 15 ppm. The first thing I noticed is that, with no CO2 on, the amount of dissolved CO2 rises back to around 3 ppm by the time I turn the lights back on, then drops down to around 1 ppm by the time the lights go off. That means the plants were using up the CO2 early in the photoperiod, something Diana Walstad also discovered in her research. (Her solution was a long rest period between lights on intervals, so the substrate could generate enough CO2 to restore the 3 ppm.)
With the CO2 on all day, the amount of dissolved CO2 goes to around 6-8 ppm. So, my improved growth of Hygro is caused by adding less than 10 ppm to the water. The other plants in the tank also are growing and looking much healthier with the CO2 on.
This was found to be the case years ago when DiY CO2 was first used. But, people continued to want more and more, so they increased their light intensity substantially, along with the CO2, and found that if they had 30 ppm the plants would grow much better at high light, and the fish would not be killed by the CO2. We then seemed to forget about the original benefits of CO2 on low light tanks.
I'm posting this to suggest that we stop telling folks that they need pressurized CO2 if they want to see any benefit in large tanks. It just isn't true. If we stick with low light, probably up to at least 40 PAR, we can gain big benefits for almost trivial additions of CO2, as long as we also dose Metricide or Excel, to stop the fluctuations in CO2 from triggering BBA attacks. This will greatly improve our success rate with our low tech tanks, and increase our enjoyment of the hobby.