Skimmer is not particularly good in removing large particles...
They are usually to heavy for the foam to lift it out of the water.
Skimmer is mostly used to remove dissolved organic impurites and
mikroscopic particles (including fito-zoo-plankton, unfortunatelly).

Skimmers do not remove ALL organic compounds. Skimmers are ONLY effective
at removing dissolved organic compounds that contain a hydrophilic and
hydrophobic portion.