In nature, all plants need carbon dioxide to produce the sugar they need for survival. As a waste product, they produce the oxygen needed for animals to breath and survive. This process, known as photosynthesis, also takes place in an aquatic environment. There is generally little CO2 in an aquarium, jeopardising the health of plants and fish and at the same time encouraging the growth of algae.
This is why you need to use artificial methods to introduce CO2 into the aquarium. There are various ways to produce carbon dioxide with either natural fermentation or a cylinder and a range of accessories to stimulate the process and optimise distribution in the aquarium.
The important thing is not to exaggerate to avoid creating even greater damage, particularly to the fish. You should therefore establish the right quantity of CO2 through tests which detect its presence in the aquarium.