CO 2 needs differ depending on the type of plant. Slow growing plants often require little CO 2 (eg Anubias). Fast-growing plants such as the beautiful green Cabomba or the decorative red Nymphaea (Nymphaea lotus) require a very high supply of CO 2 .
A healthy biotope in the aquarium requires fast-growing plants, which is why regular CO 2 fertilization is so important.