You need to measure the amount of co2 thats going into your tank, to much and you will kill your fish they will overdose on the co2 at high levels, so be careful.
You need to get a Ph test kit and a Kh test kit (the Kh kit will normally be a Kh/Gh kit) and this will allow you to measure the amount of co2 you are putting in your tank, ideally a level of 20-30ppm is required.
Also watch your mix if you are doing a Diy mix as again to much and you can cause a crash in your Ph with adverse effects to your fish.
An airstone isnt a particularily effective method of getting the co2 into your tank, as the bubbles just rise to the top and disappear.
Probably the best way other than making some sort of reactor with an airpump is to use the Hagen ladder, these just stick onto the side of your tank but they are very effective and you can still make your own DiY mix you dont have to buy premixed sachets, this is probably the most cost effective route but the point is they work ( personally i find the DIY mix much better than the sachets that come with the set) 1 Hagen ladder will be enough for a 20 gallon tank, add more if your tank is bigger.
Search through the threads in the plant section for more info on DIY mixes etc or if you have any questions on your co2 levels ask in there, most of the people in there use some sort of co2 in their tank, but the Hagen ladder is very popuar.
EDIT: Oh yeah one thing i forgot to mention in order to keep the co2 from disappearing to quickly from the water you have to reduce the surface agitation usually caused by your filter or airstones on pumps etc
