How do you vary CO2 concentration in water?

Take an experimental set up where a water plant like Elodea, is submerged in water. i wish to find out how differences in CO2 concentration will affect photosynthesis rates. I will be measuring the rate of photosynthesis by measuring the volume of air produced in a time limit. The actual experimental set up is rather difficult to explain, so i wont do that here. All i need to know is how you would vary the CO2 concentration in the water. Is it possible to do it with NaHCO3? if yes, then how?