There is no point adding carbon dioxide (CO2) to an aquarium unless it has lots of live plants, aquarium plant fertiliser, and lots of light.
CO2 is produced continuously by filter bacteria and the fish in the tank. It is also in the air we breath and that air comes in contact with the aquarium water and introduces CO2 into the tank that way.
High levels of CO2 is harmful to fish and all animals and should only used in heavily planted tanks that don't have many fish in. High levels of CO2 will kill all the fish in the tank.