Plants, of course. You should know Mgamer.
I started with CO2 with an Idea to breathe into a plastic bag and instal and air pump in there to go to the tank. But I realized that DIY is easier.
co2 isnt alway a must.
http://www.aquariacentral.com/forums/showthread.php?t=136668
what type of set up do you have? tank size lights plants all that..... you prob cna get away with out it.