All plants require a source of carbon and the 500 ml container of Flourish Excel is a simple source of readily available organic carbon that directly benefits the growth and overall development of a wide variety of aquatic plants. Typically, carbon comes from CO2 but it can also by derived from simple organic compounds such as photosynthetic intermediates, which are found in this formula. Now this isn't to say that using one would negate the others efficacy, on the contrary! Due to the fact that processes surrounding the production and development of photosynthetic intermediates occurs simultaneously, aquarists are able to achieve substantial benefits when using both Seachem Flourish Excel in addition to in-tank CO2 injections. The combination of these two carbon-based additives is well suited for situations when continuing to add CO2 could result in dangerously low pH levels within the aquarium. Additionally, the solution has efficient iron reducing properties which promote the ferrous state of iron (Fe+2), which is more easily utilized by aquatic plants than ferric iron (Fe+3). CO2 is needed to produce longer chain carbon compounds known as photosynthetic intermediates that contain compounds such as ribulose 1,5-bisphosphate, and 2-carboxy-3-keto-D-arabinitol 1,5 bisphosphate. In layman's terms, Seachem Flourish Excel does not contain these exact compounds, but ones that are quite similar, allowing you to bypass the involvement of CO2 by introducing already finished, structurally similar compounds to the benefit of your delicate aquatic plants.Directions:On initial use or after a major (> 40%) water change, use 1 capful (5 mL) for every 40 L (10 US gallons) of aquarium water. From there, use 1 capful for every 200 L (50 US gallons) daily or every other day. For smaller dosing please note that each cap thread is approximately 1 mL.