well after reading ur post i became interested in this and im sure others will to. I learned that "Protein skimmers work for marine aquariums only because salinity increases the surface tension, which is needed to attract the surfactants. In order to create the surface tension needed for freshwater, the apparatus would have to be under very high pressure." (http://www.algone.com/protein_skimmer.php)
But the most interesting thing i found is that they make protein skimmers specially made for freshwater, here is link to a sight with info on it, i'll do more research when i get a chance and see if i can get the prices these bad boys go for, but anyway heres the link http://www.tmc-ltd.co.uk/commercial/freshwater-skimmers.asp
the 150 model would cost u Total: £269.49, which translates to 425 dollars give or take, its honestly somethin u wouldn't need in freshwater tank but more of a luxory if anything