The sump! There it is.
Still, a half-pound per day of CO2? Seems like that would get expensive. Would it make more sense to simply grab a needle wheel pump and inject it separately? Just straight into the tank? It would pay for itself in about 14 months just in CO2 refills alone.
A buddy of mine was going to use this system and force the water and CO2 mist through a water filter in order to achieve 100% dissolved CO2. I have to check in with her to see if she was successful. A system like that would be, theoretically, "sump-proof," agitation-proof, etc. and a 20lb CO2 cylinder would probably last someone like me for a year and a half. I would love to see that in action!!