This will take some explanation, so bear with me.
The aim is to have a balance between light and the 17 essential nutrients. Plants photosynthesize (= grow) only when everything they need is available, and if it is, they will photosynthesize to the max until something is no longer available. What we term the limiting factor to growth. This balance can be at any level, but it is always best to have light as the final limiting factor. If light exceeds what the plants can use, which means that some other factor (a nutrient) is no longer available, algae will take advantage. More on algae later.
Carbon is one very essential macro-nutrient, but so is nitrogen, oxygen, calcium, and some others, plus there are the micro-nutrients or trace elements. And all of these have to be in relative proportion. [I won't here go into the detrimental effects of not having nutrients in proportion to each other according to the plant's needs.] So adding CO2 beyond the level where some other nutrient is no longer available, is not going to be beneficial to the plants, any more than adding more iron or copper. I don't add CO2, but I do add other nutrients because some are lacking in my situation [I have very soft water]. I also want minimum light due to the fish [read more on this here: http://www.tropicalfishkeeping.com/f...er-fish-81982/
]. And there is a lot of natural CO2 in a healthy aquarium, more than many realize. If I were to add CO2, I would have to increase my light intensity, plus start increasing other nutrients--all to balance at a higher level.
Finding the balance sometimes takes a bit of experimenting. Assuming one does not intend to add CO2, then the first experimentation is with the light; once you have the minimum intensity for the plants you want to grow, the duration can be extended or lessened. Algae is the main guide to this; if I see algae increasing, I reduce the light period. Next I fiddle a bit with nutrients, using Flourish Comprehensive Supplement (liquid) in all tanks, plus the Flourish substrate tabs in some tanks. The tabs depend upon the plants; my large swords are heavy feeders so they get the tabs.
Back to the algae; algae is natural and will be present in any healthy aquarium. In one with live plants, we want to keep algae in check, only because it can attach to plant leaves and suffocate the leaf and the plant. Aside from this, there is no detriment to algae. It will always be there, in some form (there are many types, and some occur more than others in this or that aquarium), but it shouldn't be allowed to proliferate. I let it grow on wood and rock. I keep it off the glass mostly; cleaning the front glass especially at every water change is a good idea, even if nothing shows. But remember that algae is only there because there is more light than the plants require in balance with the nutrients.
You may have more questions from this, so I'll end here for the present.