protein skimmers

salt content mixed with air and waste causes the foam to form. The foam is suspended waste.

Ditto^

^ right. The organics are usually relatively insoluble in seawater to begin with (same with freshwater, but much more so). Also, many, if not most, are hydrophobic (water hating). These molecules prefer to either be in air or, if exposed to water, prefer to be at the air-water interface--in other words, it is more energetically favorable for them to reside at the surface of the water. That explains the film. As for skimming, the many bubbles created make one of the same air-water interfaces. By making many, many small ones, the skimmer simply takes advantage of the natural, energetic tendencies of these molecules and increases the overall surface area. A relatively simple (but not entirely understood) process that is effective at removing hydrophobic and amphipathic (both hydrophobic and hydrophilic [water loving]) molecules. Hope that clarifies it a bit.

If you would some additional reading try this site: http://www.reefland.com/rho/0505/medprod4.php Has a lot of good concise information on skimmers and their process. HTH

I actually found this article by Randy Holmes-Farley, which goes into greater detail about what I was touching upon. It even goes into the "trace element" debate (i.e. do you need to add more [the answer is a resounding 'no'], ozone, iodine, etc.