Aquarium plants photolithoautotrophs, meaning they assimilate inorganic carbon (CO2) using the power of light. They do this a as method of making sugars like glucose which are then used in the same way as other organisms like you and I.  When keeping aquarium plants, carbon is often a limiting factor. Without supplementation, carbon is often in very low concentrations. This will in turn slow down their ability to remove harmful compounds like ammonia as well as other nutrients that can lead to algae problems. Many aquarium plants have developed strategies to overcome carbon limitations in the freshwater environment by using a different, and often more available form, being bicarbonate (HCO3-).

 

     Some plants are better than others at using bicarbonate as a carbon source while some species may not be able to use it at all. In general, plants that evolved in hard water environments where GH (calcium and magnesium) and KH (carbonates) factors are high typically have an enhanced ability at using this carbon. Most plants studied, even those that come from soft acidic waters with low KH display the ability to use carbonates. Despite all this, aquarium plants as a whole will always benefit from having additional CO2 as it is the prefered carbon source.