Thank you very much, I still have to read the article but this is a data that I've been looking for ever since, I found a lot of articles about Oxygen content inside the water but nothing about CO2 related to plant growth. I did myself some checking in Italian rivers, lakes and even sea; of course my testing was "ridiculous" if we are talking in terms of scientific precision but that was not my goal. I just needed to have a rough idea of what the natural CO2 content was in natural basins of water and compare the natural environment with our aquariums, where we want our plants to grow and be in best conditions. We consider the right CO2 amount in our tanks to be around 20 mg/l up to 30 mg/l but this is something that almost never occur in nature. The highest amount recorded by me was on the sea shore where I found an exceptionally high CO2 content of 9 mg/l which was a surprise for me and it was higher than in comparison the freshwater rivers and lakes where I found values around 5 to 8 mg/l. This year I will do it again.
But the real big question is: if in captivity most demanding plants require at least 20 mg/l of CO2 to grow where do they get it from in natural environments ?