OP, if you are located in chicago, like your location says, you should have no issue getting CO2. Im origionally from chicagoland, I used to do planted aquariums and had co2 injection systems on them, using small 5lb co2 tanks and for calcium reactors on reef tanks. You can get them refilled or exchanged at probably any welding supply store. I forget the name of the place but I used a welding supply company I believe it was in evanston or near there anyway.