During photosynthesis, plants use light energy to capture CO2. This CO2 is used to build the basic carbon structures from which all plant material is made. In a poorly lit aquarium, light is likely to be what limits the rate of plant growth. The amount of Co2 produced by fish- and bacterial respiration is more than enough to allow photosynthesis under these conditions. If on the other hand, you try to make your plants grow faster by adding more light, it is likely that there will not be enough CO2 in your aquarium. The plants simply can not grow as fast as they would like to, given the available light energy. The easiest way to increase the amount of CO2 in an aquarium is to buy a CO2 gas bottle and let it bubble into the water.

100% CO2 gas bottle  UNI EN ISO 11118