Also to add.....from what I have read and from observing the popular reactions to water testing....
You cannot harm your tank inhabitants with CO2 water measurements on up to 30 ppm and probably more. In the natural bodies of water many of these plants inhabitant (even if just temporary), the CO2 measurement in the water is from 15 ppm to 20 ppm. Most are satisfied with this measurment, although you can certainly add even more. With the lights providing the lighting on most tanks, however, more isn't necessary. If you're not providing more light than the sun provides in nature....more CO2 than what is in nature probably wouldn't do any good.
So far I have a lot of plants prospering in my tank, but I have easy to grow plants. So far they are just cabomba and amazon sword. I have about 1.5 wpg which is ok for what I have and so far.....CO2 has not been necessary...though I'm sure it would have helped more than not having it.
If I were to be providing 2 wpg or more (which I will be soon), I would definately add CO2.
Buy a water test kit that measures CO2 in your water before you begin this...so that you know where you stand even before you add any CO2. I plan on having a moderate to heavy fish load of active fish......so I may not have to add quite as much CO2 as someone else with my size tank.