With the lights out is when the microscopic critters and other things that would normally get eaten during the lighted periods (also their are more spawning type events) thus the skimmer gets more organic and inorganic materials going through it. So it tends to go into a hyper state.
To tune it, squeeze the air tube until their is no air going into the skimmer, from their adjust your water level so that it sits right at the black ring level. This is a good spot to start fine tuning. Remember nothing in a skimmer reacts right away, it could take hours for it to hit a constant state. SO once you get the water level their turn the air back on, it should bring the water/air bubble mass up to the top of the twist off ring. Let it be their for a couple hours and then check it again and make any fine tuning you may need to.
hope it helps
Mike