Plants require carbondioxide to prepare food through photosynthesis. Hence, maintaining adequate levels of carbondioxide is essential in an aquarium with plants.
Carbondioxide occurs naturally in the aquarium from fish respiration and other microscopic processes. This is sufficient for plant growth. However, some hobbyists interested in faster and richer plant growth add carbondioxide to the water.
Carbondioxide can be added by a simple home-made yeast fermentation bottle. The outlet of this bottle can be fed to the inlet of a power head.
There are ready-made carbondioxide reactors available in the market.
Some pet stores offer carbondioxide tablets which is to be dropped into the water. These tablets release water over a short period of time. The effects of these tablets is debated.