Foxface, Try adding a air pump and a wooden air stone.
Run the air line in through the lid and down toward the bottom of the skimmer this will add twice the micro bubbles.
just make sure you can adjust the air flow!
I did it to mine and it made a big difference, also check the air inlet tube on your pump, mine gets a build up in side of it that I have to clean once a month with a small drill bit!!
Hope this helps..