09-08-2010, 12:13 PM
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I wrote on this not long ago, but as I can't find it, I'll try to summarize.
Aquatic plants will live in almost any environment (here meaning substrate, light, water parameters). But growth (i.e., photosynthesis) will vary greatly. I have fish aquaria that include plants, so my approach is different from an aquarist who wants a plant tank without fish, or with few fish. Each of us will approach things differently because we are after a different goal.
As I frequently mention, plants will only photosynthesize if there is a balance of light and nutrients sufficient for their individual needs. One macro-nutrient needed by all plants is carbon, usually assimilated via CO2 though some plants are well able to assimilate it via bicarbonates. But carbon is necessary for photosynthesis to occur. Different plants have different needs respecting light and nutrients. But no matter what, these must be balanced for the specific plant.
Once the balance is achieved, then comes the level. Plants will "live" at a low level if it is balanced. I had a 33g running for several months with no fish, just "spare" plants. The light was on normally (all tanks are on the same timer) and I dosed Flourish once a week. There were no fish in that tank. The plants did well, better than I would have expected. Obviously their need for carbon came from somewhere, presumably the bacteria and Malaysian livebearing snails, and from the air at the surface (esp the floating plants). The plants in this tank were not as robust as those in my main tanks--which is to say they did not grow as fast or send out runners as much--but they were green and alive.
The level at which each of us wants the plants to grow will determine what we have to provide. Or in reverse, the plants will respond with growth that is dependent upon what we provide, by which I mean light (intensity and duration) and nutrients. Some plants demand higher levels of these, but many will exhibit various levels of growth in relation to the level of light/nutrients we provide.
To say that plants cannot grow in an aquarium without added CO2 is simply false; not that you're saying this, but I know there are some "authorities" who do give that impression. There are a few plants will not do well, and some others may have very slow growth--though increasing CO2 itself will not help if the light and other nutrients are not increased to balance. But the majority will manage fine with the carbon available via CO2 from the fish and bacteria--and there is considerably more CO2 from bacteria than from fish.
To end with an example: the extremely high-tech setups of Takashi Amano that were discussed in another thread earlier this week require mega-light (4-5 times the intensity I have), CO2 diffusion, and daily injection of copious amounts of fertilizers to provide the nutrients to balance. These tanks are very high maintenance, and very expensive to setup and operate. And there is a considerable amount of intervention by the aquarist (via equipment and maintenance) in the normal natural processes that result in a healthy aquarium. By contrast, I do nothing more than a weekly partial water change and dose a minimal amount of liquid fertilizer twice a week. The plants I select do well in this minimal approach. Each aquarist has to decide the level in plant growth that he or she wants and then be prepared to provide what is essential to achieve that level and keep it stable.