quite a few people here are
i have two tanks wit them now. and im one step past sump, i have a wet/dry filter which does exchange co2 fairly readily.
if you set your sump up properly, you will increase oxygen levels and still retain much of you co2, such that the 5 pound tank will last maybe say 4 months?
not bad considering my 20 lb lasts me 2 months on my 75 gallon