If you've ever been to a beach at low tide and seen (and smelled) the brown foam that accumulates on the shore, that's pretty much what it pulls out of the water. The nice thing about a skimmer is that it actually removes some contaminants from the water as opposed to just trapping them in a media that's still in the water. However, I agree with Gti_Leo that it's not a necessity on a FOWLR, especially a small one.