Carbon is the backbone of all life.
Every organic molecule of every living organism is predominantly carbon based. Given this simple fact, it becomes clear why carbon plays a pivotal role in the planted aquarium. 
Aquatic plants extract CO2 (carbon dioxide) from their environment and employ it in a process called photosynthesis. Photosynthesis combines CO2, water and light energy to produce simple carbohydrates and oxygen (O2). The first and simplest carbohydrate produced from photosynthesis is 3-phosphoglycerate. It is from this simple molecule that larger and more complex carbohydrates arise (by way of a variety of enzymatic processes).
Growth rates of aquatic plants are strongly correlated with availability of carbon and the plant’s affinity for carbon uptake. Studies1 have shown that plants with the greatest carbon affinity have the greatest growth rates, whereas those with lower carbon affinity have correspondingly slower growth rates. Because carbon availability is normally the limiting factor to growth,addition of CO2 to a planted aquarium will always result in large increases in growth (assuming other critical elements are not lacking).
Flourish Excel™ provides a simple organic carbon molecule that plants can use as a building block for more complex carbohydrates. Because Seachem’s Flourish Excel ™ is an organic carbon source it does not impact pH. The chemical structure of Flourish Excel™ is quite similar to some of the products of photosynthesis such as Ribulose 1,5-bisphosphate and 2’-carboxy-3-keto-Darabinitol 1,5 bisphosphate. Flourish Excel™ possesses the same basic 5-carbon chain seen in these molecules