http://reefkeeping.com/issues/2005-08/eb/index.php
"Figure 1 shows the oxygen dynamics of Tank 1 in operation without a skimmer. Figure 2 shows the oxygen dynamics of Tank 2 with a skimmer in operation."
"I had assumed (wrongfully) that oxygen was maintained at high levels through the use of two powerheads that agitated the water's surface. However, once the lights went out and photosynthesis stopped, oxygen levels dropped quickly from a high of 78.7% of saturation to a hypoxic low of 16% of saturation. "
Notice the skimmer took the tank from 16% to about 80+% with only the addition of a skimmer and all else constant in Tank 1. In Tank 3, the tank wasn't oxygen starved at any time so it was more difficult to see the effect of the skimmer since his tank was never low in DO even at night. It would have been useful to see the effect of a skimmer on a large tank that has a low DO level at night.
I think some people have low DO levels at night and never know it. It seems that clownfish can survive at 16% for some time whereas I'm quite sure angels can't. This explains why I've lost angels the morning after I forgot to turn my venturi back on in a skimmerless tank.