I use Pure Energy 20oz tanks. On my 98 and Alpha Black I get about 1,000 shots out of them. On my A5 with my cyclone and response running, only about 750 or so.

The only good thing about Co2 tanks compared to HPA tanks is that they are considerably cheaper priced. Other than that, Co2 causes your guns to be less accurate, your shots are less consistant and also causes your gun to malfunction periodically when playing in the winter.