One problem we all have when we write up a primer on a planted tank subject is that over time the "facts" are found to not be facts. Some "facts" go from "you can swear on it" to "well perhaps under some circumstances" facts. CO2 is just one more such subject.
Years ago, the old timers learned that they could improve their aquatic plant growth if they could just get more CO2 into the water. Yeast/sugar DIY CO2 soon followed, and that method is still in use because it worked for so many different people. This encouraged people to try for still better/faster plant growth by using still more light. But, that caused severe algae problems to erupt.
Then the old timers started looking for a better way to get more CO2 into the water to help avoid the algae problem. One way that was tried was dry ice - never did work well. Another (tried by Amano) was pouring carbonated water into the tank. That worked but was super expensive and a lot of extra work.
Finally pressurized CO2, using bottles of liquid CO2 as the source, was developed into the ultimate, best of all methods for using CO2. And, that opened the door to using a lot more light, making a lot more species of plants growable in our tanks. It was soon found that if you kept about 30 ppm of CO2 in the water you had an acceptable risk of killing fish, good growth of most plants, and the CO2 usage was low enough to keep the costs and labor down. So the "holy grail" of using CO2 became 30 ppm.
But, we forgot something: DIY CO2 did work, and was very effective for a lot of people, using not just 10-20 gallon tanks, but even bigger ones.
In 2002 Ole Pedersen and others, in Denmark, wrote a scientific paper about Liebig's Rule of the Minimum, including CO2 and light, which contained a lot of useful data that explained why DIY CO2 worked so well it is still in use. In 1999 and later Diana Walstad wrote her book about her method for keeping a very low tech aquarium. In that book she included some information about the role of CO2 in a natural tank. That also was a good clue about why DIY CO2 had been so successful when first tried. But, we planted tank people were still concentrating on growing ever more light demanding, CO2 demanding plants, and we ignored what all of that information should have taught us. So, the "fact" that "you can't use DIY CO2 successfully on big tanks without multiple big bottles of yeast/sugar solutions" became unquestioned, and continues to be today.
The data in the Pedersen paper was a compilation of growth rates versus light intensity and CO2 concentration, for one plant - Riccia. That data can be plotted to show that for light intensity between about 25 PAR (very low) to about 90 PAR (very high), for Riccia, it only takes about 10 ppm to get the maximum growth rate for Riccia. More than 10 ppm does not increase the growth rate!
Diana Walstad was able to improve her plant growth by the simple method of having a long dark, resting period in the middle of the photoperiod. That allowed the natural production of CO2, by the substrate, to restore the concentration of CO2 in the water, after the growing plants had depleted it. She was dealing with less than 10 ppm of CO2 maximum.
I have found that Hydrophila cormybosa simensis, a low light, fast growing plant (unlike the slow growing Riccia) will grow very much faster with DIY CO2 at a concentration of less than 10 ppm. This was done with a 65 gallon tank, with about a one bubble per second production of CO2. Other plants in the tank, including Bacopa, crypts, vals, etc. also grew much healthier and faster with that small addition of CO2. So, my experience backs up the data from Pedersen and the method used by Diana Walstad.
It is time to stop telling people that DIY CO2 is useless on big tanks. For low light tanks, 30-40 PAR light intensity, if not more, DIY CO2 from a one bottle system can be very useful on tanks at least as big as 65 gallons. There is no reason why it would not be useful on much bigger tanks.
Today we can use a citric acid/baking soda DIY system, which is controllable, unlike yeast systems, and have a very cheap, very beneficial addition for our big, low to low medium light planted tanks.