What kind of lighting do you have?
What kind of filtration?
Are you running your lights and CO2 on timers?
What kind of plants?
CO2 is usually the limiting factor to plant growth. What I mean by limiting is that even with high lights and ferts if the plants are not getting enough CO2 they cannot properly utilize all of the light and ferts. When you add proper amounts of CO2 it will allow the plants to use more light and ferts for growth and photosynthesis.
Things to keep into account with CO2
1) Too much can "gas" your fish which means they can basically suffocate if you use to much CO2 and don't have enough oxygen.
2) If you have bubblers or HOB filters you can loose a lot of your CO2 to surface agitation.
3) You need to address how you will get the CO2 into your water. If you have a small tank then a simple diffuser will work fine but if it a large tank >75gal then the diffuser will not be as affective because most of the bubbles will reach the surface and release into the atmosphere before it is absorbed by your tank.
4) CO2 can drop your pH significantly because CO2 is a weak acid.
5) Big swings in CO2 can cause a lot of algae growth so you want to shoot for stable levels of CO2.
Benefits of CO2
1) limiting factor to plant growth is removed.
2) Assuming you have adequate lighting and ferts your plants will respond very will to CO2 and you will see growth much faster.
3) Again, assuming you have appropriate light/ferts your plants will grow thicker and you can grow more demanding plants.
4) CO2 will allow you to run more powerful lights for longer periods of time while controlling algae in your tank. This is because the CO2 will allow plants to out compete algae in your tank because they can utilize more of the nutrients which will decrease the amount of nutrients available to algae.
I hope this helps. I have been running high lights, pressurized CO2, and dosing ferts in my tank for about three months and the growth is amazing. I dont want to post pictures and high jack your thread so just check out my tank in my signature line and that will give you an idea of the growth rate you can expect with high light, CO2, and ferts.
I wish you luck!