I'm not looking for a commercial solution. I'm just looking at the logic for a home brew for the best method.
Surely the CO2 stops the O2 getting in so it's just dissolved O2 that is involved anyway.
I've been thinking a bit more about this. I don't know the answer from experience but here's some logic... The CO2 released is heavy so it says on the surface. Open or closed, without an air pump no O2 is getting in. So any argument about keeping it open or adding air needs to allow for the fact...