Carbon is just one of the many nutrients plants need. It can come from the substrate, but only in limited quantities, so substrate CO2 is fine for low light tanks, but not for tanks with high lighting, where the plants are forced by the light to try to grow much faster. Flourish Excel is a source of carbon for the plants, but not at the same level as we can get CO2 to in the water. I suspect that is because Excel is toxic if you tried to jack up the level that high. So, Excel is good for relatively low light tanks. CO2 is needed for high light tanks, because we can supply enough to not limit the plant growth. But, soil as a source of carbon is used in all tanks if it is there. Excel will add some carbon for all tanks. And, CO2 adds carbon for all tanks. As long as the level of CO2 in the water is not so high as to harm the fish, it is fine for any tank, and as long as Excel is not dosed too much beyond the bottle recommendation it is fine for all tanks. All of this being my opinion, of course.