Aaron,
If Excel/Easycarbo was only pure carbon then we wouldn't need to spend £20 per liter. If it was only carbon it wouldn't be so incredibly toxic to us. We would just need to throw some charcoal from the fireplace into the water.
The chemical process of photosynthesis and the physical construction of the reaction chambers, as well as the transport mechanisms (Rubisco enzyme) specifically requires Carbon in the form of CO2. Plants use the Calvin Cycle to "Fix", or assimilate Carbon from CO2. The O2 is simply a waste product of that cycle. Some of this O2 is used in "glycolysis" which is the process in which cells respire and consume energy.
Excel/Easycarbo is a complex Carbon compound constructed of a molecular polygon which includes Glutaraldehyde (C5H8O2) as it's foundation. Many higher plants have the ability, through other chemical reactions to break down this complex molecule into CO2 which is then transported to enter the Calvin Cycle. Some plants, like Riccia, and most algae do not have the facility or the sequence of required sophisticated chemical reactions to perform this breakdown and so the Glutaraldehyde becomes toxic. It's therefore a gross oversimplification to describe Excel/Easycarbo as "just Carbon and nothing else". Far from it mate.
Cheers,