I got the idea of what it was, but never used one so didn't want to steer you wrong. It says right on there to install in the outlet side. That would have been my guess, for no other reason other than I don't know what the filter media and bacteria would do to or with the CO2. At the very least I would think it may suffocate some of the bacteria or possibly use up a small amount of the gas. Just speculation though. Hope it works well for you.