The typical "off the shelf" model protein skimmer would not work properly, if at all on a fresh water aquarium. It would not foam up like it dose in saltwater.
In order to work on freshwater, the water injection would need to be very high pressure to develope enough (and fine enough) bubbles to produce a foam that would remove organics from your water. THe typical ventiru might not work under this type of application either, the water pressure might push water backwards thru the venturi air inlet.
Protien skimmers are defenitly worth the money and need little maintenance to keep them operating correctly (assuming it's a good model) but for fresh water you most likly would need to either build your own or have one custom made.
Joel
