To me, the most important are of a tank to filter is the waters surface. The air water interface is where the most toxic, water degrading stuff builds up, (proteins, other dissolved organic carbon, etc). But because most of this stuff is invisible (unless its oily or scum) it isn't thought about.
I have seen surface skimmer attachments that would deal with this stuff, and at the same time eliminate all but the most light weight sand form entering the filter.
Sure everyone thinks fish feces is a filters main target, to me it is not, that's what vacuuming is for.