You don't have nearly enough CO2 to make any significant difference. And, if you really do have 80 watts of Fluorescent light on a 20 gallon tank, you need CO2. Are you using screw-in power saver bulbs? I ask that because 6500K is a standard number for some of those bulbs. If that is what you are using, be sure to ignore the big wattage numbers on the package, and look for the actual power that the bulb uses, which is usually an odd number, like 27 watts, or 23 watts, or 17 watts, etc. Those are the numbers that count.
Plants need nutrients to grow, so you need either a nutrient rich substrate, like ADA aquasoil, or mineralized topsoil, or you need to use substrate fertilizer tabs, or you need to add fertilizers to the water. Which are you doing?