Higher CO2 concentration levels allow the plant to put on more weight faster, and it allows you to use higher levels of lighting, which will improve the rate of growth, however there are drawbacks. As the CO2 concentration level increases the CO2 uptake efficiency of the plant DECREASES. That's because the plant knows that it can now allocate more of it's resources to other things, such as pigmentation, Nitrogen uptake and so forth. The CO2 uptake system is a very expensive, high maintenance system.
Having purposely reduced the robustness of it's own CO2 uptake system, ironically, the plant now becomes more susceptible to CO2 shortfalls. So when the tank is at very high CO2 levels, even minor downward fluctuations of the CO2 concentration will have a major negative effect on plant health, exactly because the CO2 uptake system has become flimsy. At lower CO2 concentration levels, the plants knows that it must dedicate a large proportion of it's energy to ensuring CO2 uptake, so at low levels it can more easily tolerate larger percentage gaps, drops and fluctuations.
CO2 therefore, is as Barr described it - a narcotic; the more you feed them the more you'll need to feed them.
This is exactly why people who decide to add CO2 to their tanks immediately begin to suffer CO2 related deficiency syndromes. Then they go into denial because they think: "but I'm adding CO2 now so how can melting and hair algae possibly be due to CO2 deficiency?"
