I can tell you from experience that General Air and Gas will exchange older non CO2 canisters for CO2 however they usually will offer an exchange of one size lower than what you have to trade if they dont happen to carry the same size. For example I had an old 30lb tank that they exchanged for a new 20lb tank because they dont sell 30lb Co2 tanks.
Thanks for the information. I am considering renting from General Air and Gas, the prices seem quite tempting. It would be nice if I can find a deal here though.
thanks opakapaka! I am looking for mainly just a CO2 tank and regulator so that I can grow my phytoplankton to be more nutritious! I'll see if they'll give a college student a deal.