There is no leftover CO2. At 4-5 bps it's completely dissolving. I don't know what the capacity is because I'm not interested in putting my fish to sleep. There is NO gas standing around at the top of the housing. I'm also using the venturi intake that comes with the Mag-5 with the fractionator impeller. I'm using a solid impeller on it though. I don't know if that makes a difference or not. It's nowhere near maxed out as it is.