Bob, I believe O2 goes in by itself quite easily once CO2 is dispelled. The skimmer does an excellent job of agitating the water that flows through it, and I believe this does in fact provide for a higher O2 level than if a skimmer was not used.
Good ciculation in a tank with the sufficient surface agitation could provide the same amount of CO2 off-gassing, but this would require a lot of water movement.
Ultimately there is a maximum and saying that a skimmed tank will always be higher in O2 is certainly not true. A skimmer will not supersaturate O2 in to solution. It does provide an easy way to remove CO2 though.
So, required? No. Helpful in this task? I believe so. Again, so supporting research, but the logic is sound.
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Brad
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