gorally,
This is taken from the pdf under the section of Commissioning
Turn the air adjustment screw (1) open completely.
It is possible that a newly operated skimmer requires
a start-up phase of a few hours up to a day. In case
of aqueous foam the air adjustment screw must be
closed somewhat
http://www.tunze.com/fileadmin/gebra...x9004.8888.pdf
So my interpretation of the above is you start full air and close by increments. It's possible that if you have left the skimmer to dry out. It will need time to slime over again before you achieve zero bubbles.