Wet wet wet skim!

The air adjustment nob is at the end of the air intake from the pump (which goes to the top of the skimmer cup). It looks like just a black cap with a hole. Lighten or loosen this in order to control the amount of air allowed into the skimmer.

She has the Hydor Slim Skim. Water depth/skimmer height can be adjusted by using the included magnets which attach to the rear of the skimmer. That's the only big problem I've found; you can't easily adjust skimmer depth in tight spaces (Hydor says the magnets/clips can be removed to reduce skimmer footprint. But then you need to put something uder it to adjust depth).