Originally Posted by New2Betas
So what kind of tank would use co2?
First one has to understand the basics. Plants need light of adequate intensity and duration, and they need nutrients. There are 17 nutrients, one of which is carbon (mainly assimilated from CO2). All this has to balance. If it does, the plants will grow (photosynthesize). The "balance" can be any several levels, from minimal (low-tech or natural) up to high-tech. The level you want to maintain will usually depend upon your idea of what a planted tank should look like. In the most basic of terms, my tanks are low-tech; Takashi Amano's are very high-tech, as are the tanks commonly called Dutch Aquarium, where plants are the main focus and some do not even have fish.
We know that in their habitat, most of the plants we keep use fairly low light; many never see the direct sun. They grow relatively slowly, but they are healthy and growing. My plants are like that. The growth is "slow" though sometimes I find it hard to imagine faster growth; I know I certainly do not want faster growth because the plants are serving their purpose very well at the rate they now grow. Increasing light allows for faster growth. But only if the nutrients are also increased in balance. This is where CO2 comes in.
Amano's method uses very high light (4-5 times what I use), CO2 diffusion, and copious amounts of fertilizer added every day, not just once or twice a week as in my tanks. The plants look lovely in both, as far as I'm concerned. But the high-tech approach requires a lot more investment in money and maintenance. Using fertilizers daily costs money, as does the CO2, and the increased lighting will really raise your hydro bill. So it comes down to the "look" you want and what you are prepared to invest in cost (initial and operating) and maintenance.
The light is the big reason I do not advocate high-tech. This affects the fish. Most of the fish we maintain in planted tanks occur in very dimly-lit waters. This is what they are used to. Given the option in a large aquarium, such fish will choose the shaded areas. I see this all the time in my tanks. As just one example, cardinal tetra have what Baensch/Riehl term a light phobia; they will always be under plants in the lower third of the aquarium, and because they choose this due to their natural instincts, it means they will be healthier. We can be guaranteed of that. I prefer going down that road, of providing the fish with as natural as environment as I can, because I believe (and my 20 years experience corroborates this) that the fish will be healthier and live more normal lives. There's more to this than just light: the filtration (water flow) is critical, as are water parameters, plants, wood, etc. My cardinals which also do not like moving water, remain in the right half of the aquarium at the opposite side from where the outflow from the filter is placed. Some of the Corydoras species prefer water flow and always settle at the end under the filter outflow; other species come from quieter streams and have settled further down the tank under wood where the water is still. Knowing these preferences for each species will mean a more successful community tank if the fish's needs are met.
A bit wordy, but this is to me a very important aspect of maintaining an aquarium that is healthy and successful.