Just to make sure everyone understands, Flourish Excel is not, and does not, contain CO2. CO2 is a gas at room temperature and atmosphere. Basically, the benefit of injecting CO2 is that it allows the plants to obtain carbon in high quantities. Carbon is needed in photosynthesis, and CO2 provides the carbon.
Flourish Excel is a liquid that contains organic carbon. It's another method of letting plants obtain needed carbon. Excel won't provide the plants with as much carbon as they would derive from CO2, but it provides enough to be beneficial. While toxic in higher doses, it's safe to use in the recommended dosages.