Your figures don't surprise me. You have 90gal=350L. If you run 15ppm, that is .015/1000*350*1000=5.5gCO2/tank. If you figure you need to replenish that 3Xday (gotta be about right), that is 17gCO2/day. 20# = 9000g = 500 days = 15 months pretty close by my figuring. Do you agree? Dave Gomberg, Experimenta San Francisco CA USA gomberg at wcf_com