I'm not sure a direct comparison between natural systems and a tank can be made. There are not many natural streams that have high CO2. Most are very low and filled with algae, which seems logic. Aquatic plants in those streams are mostly coverd with algae too, except for the top few inches.
We can prevent this by adding CO2. Still CO2 is a very difficult matter. There's not much too rely on. pH/KH table, drop checker, pearling, all not more then indicative. Fish? Well, we can watch the fish, but if I need 120 mg/l of CO2 to make the fish surface, (See Yme's topic) that's too much too. I don't want 120 mg/l, I want a stable 40 mg/l. I really ask myself if these levels like more than 100 mg/l are not damaging fish health. Not directly, but inderictly in relation to size and longeviness.
Better watch plants, take CO2 slowly back until you get some problems, like BBA or stunting or if you start with BBA to begin with, gradually increasing until the side effects are ellaviated. This takes weeks of course, not a few days like using fish.
I wish there was a
reliable way too measure CO2 for the hobbyist, without investing in a $$$ CO2 meter. With light I can use my PAR meter and a somewhat useful graph can be made that most people can use, nutrients is easy too, I just add more than the tank needs.
But CO2? I have good growth, and no algae except when I make mistakes myself. Still I'm in the dark. Do I have 30 ppm, 40 ppm or 80 ppm? I don't want 80. Or maybe it's just human nature to calculate everything and we have too learn too look at our plants.
It's possible. Look at the picture of the Stellata.
Beautiful plant which was over 9 inch wide. But look at the section in the middle. You can see that it's smaller than the lower and upper part of the plant.
The period in which the plant grew smaller coincided with a period of higher water temperature. With higher water temperature CO2 demand rises. Apparently the plant got somewhat limited and grew smaller leaves for sometime. I didn't get algae in this period. When temperatures dropped, it grew back to it's original size.
This tells me that I have a good CO2 level, but only for the water temperature I'm using. So I'm kind of at the lower limit. So using plants as a test works, but we have to be able to explain why things happen.
That's not easy for a lot of people. Watching your plants carefully on a daily basis helps.