Skimmers do also effect aeration/O2 saturation. After all they inject air directly into a stream of water. Can't get much more agitation than that. Just one of the many positive effects a skimmer has. Adding a sump also helps in that the water being drawn from the tank via the overflow is able to "blow off" excess CO2 on it's way down into the sump. Sumps also use surface skimmers which gets rid of the "scum" that tends to accumulate on the waters surface. That allows for better air exchange.IMO surface aggitation is what allows aeration of your tank.