08-09-2010, 06:58 PM
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If you have read the series "A Basic Approach to the Natural Planted Aquarium" stickied at the head of the plant section you will have come across the concept of "balance" which applies to any planted aquarium, high- or low-tech. The principles are the same.
Plants need a continuous balance of light and nutrients. Carbon is one nutrient they need, along with 16 others. Each of these, within a certain range, must be present along with adequate light in order for plants to photosynthesize (which is how they grow, as you may know). If any one of these is missing, the plants cannot photosynthesize fully, and growth may be restricted if not completely stopped; it depends upon the nutrient and other conditions.
Carbon is normally assimilated as CO2 by most aquarium plants; some can use bicarbonates, some can't. All can use CO2 and generally do. CO2 is readily available from the fish and biological processes, and in greater supply than many realize. If you take a look at the photos of my tanks under "Aquariums" below my name on the left, you will see what I consider to be quite good plant growth and there is no CO2 added to any of my tanks. Plant growth is slower but steady and healthy. And the light is in balance, along with the other nutrients. If not, these tanks would not look as they do.
If you want fast plant growth, then adding CO2 is one step. There are very few plants that will not grow without added CO2, in fact I doubt there is a single one; some will do better with added CO2, but the majority grow fine without it. But along with additional CO2 comes additional fertilization and more light, since all this still has to balance. The end result is an aquarium with 4+ times the light it would otherwise require, and daily instead of once or twice a week additions of fertilizer plus far more of it each time. Adding just CO2 will not help the plants if the other nutrients and/or light are inadequate for that level. It is the law of minimum as botanists call it; plants grow at the rate determined by whatever is in least supply. And when something is in excess beyond what the plants can use, algae will quickly take over.
The other issue with this is the effect on the fish of brighter light in particular. The fish we tend to maintain in planted aquaria occur in very dimly-lit waters, many of which never see direct sunlight due to the forest canopy. Some fish species are plainly stressed by bright light, bringing on health issues. I take the view that fish will always be healthier if we replicate their natural habitat conditions as close as we reasonably can. So the plants must be subservient to the fish, not the other way round.
As for adding CO2 now and then, as I hope I've pointed out above, this will do nothing but increase algae and unsettle the fish. The plants can't benefit because the balance is out.