As I mentioned in another thread yesterday, natural CO2 occurs in the aquarium in greater quantity than many realize. Fish, plants and some bacteria respire continually (24/7) and this releases CO2 into the water. But the greater amount occurs from the breakdown of organics by bacteria.
The aim is to create a balance between light (intensity and duration are both important), CO2 and other nutrients. Those of us who have "natural" planted tanks, meaning no added CO2, know that it may take a few weeks to find that balance. And of course, there are some plants that will not survive in the moderate light intensity and without diffused CO2 to balance; but a majority will. The type of plants and the expected growth rate wanted by the aquarist will determine which method is best suited.
I have always preferred less intervention in my fish tanks, allowing nature to do most of the work. Some plants thrive, a few don't; I use what works. The photos below are just two examples of what is possible with this "natural" approach; these tanks have no CO2 addition by any method, relying totally on what occurs naturally.
Byron.