Quick comment on the initial airstone question, then I'll answer the latest on how to get CO2.
As zof indicated, this is a debatable point these days. For years every planted tank author, including botanists and microbiologists, advised that any excessive surface disturbance would drive CO2 out of the water faster, and draw in more oxygen [this can actually be detrimental to plants]. Now I am seeing the opposite view. I'm going into this with some colleagues, so stay tuned.
Now, to the CO2. Those of us with natural planted tanks rely on nature to provide CO2, and this occurs in a few ways. Obviously, fish, plants and bacteria all produce CO2 through respiration which occurs continually. CO2 also occurs from the breakdown of organics in the substrate, and this is very important for several reasons. And CO2 also comes from the air (atmosphere). The latter is variable, and tied to the issue mentioned above. It can also arrive in the tap water during each water change; this too is variable as it depends upon how much dissolved CO2 is in the tap water. You can sort of test this by testing the pH of the tap water; if the result from testing water straight out of the tap is a lower pH than if you let the water sit 24 hours or alternatively shake it vigorously [both of which out-gas the CO2], that means there is CO2 in the tap water.
If you want more detail on how the natural CO2 works, check out my article on bacteria:
Bacteria in the Freshwater Aquarium
Beyond the natural processes, one can introduce CO2 via diffusion. Some of our members use this method. It raises the balance in the tank, meaning you need more intense light and increased nutrients in order to balance the additional CO2, otherwise it is just wasted. Plants can only photosynthesize if everything they require is available, and that means 17 nutrients (of which carbon is one) and sufficient light intensity. Plants grow faster the more light and nutrients they have, so this high-tech method works for those wanting to propagate plants, or maintain a few of the more demanding species.
Another carbon source is liquid. I personally do not advocate using these products, as they are chemical and do have some issues. Some plants will melt when these are used. They are toxic to fish when overdosed. One member reported irritating (sort of burning) his hand when one of these came into contact with his skin. I just don't recommend using such chemicals in a fish tank when it is not necessary.
If you'd like some more info on the whole balance concept, there is a 4-part series at the head of the Aquarium Plants section entitled "A Basic Approach to the Natural Planted Aquarium."
Byron.
Similar Threads 
