Hi, try this setting for those who is unable to achieve getting foam.
1) Turn off the air control valve.
2) Turn the water flow valve until the water rise to about 2.5cm below the o-ring ( the black rubber ring).
3) When this is achieve, turn the air control valve until you see foam forming.
4) Try to "Very Fine tune" fine tune the water flow valve.
If your skimmer is new, the first few days you will get lots of white colour foam. after that you will see less foam but more concentrated foam.
You should then get your result. For those who have salt water in the contain, this means over skimming (waste of salt water) try ajusting the water flow.
Hope this help user of prizm user.