I disagree that protein skimmers don't work on fresh water. I have been using them for years on my freshwater tanks, and ponds.
Fractionation occurs naturally in fresh water, whenever you see foam due to wave action, turbulence under a water fall, or in fast flowing rivers, that's fractionation (natural protein skimming).
In fresh water it just takes more agitation, because fresh water is less dense than salt water, and most aquarists are not willing to provide a strong enough pump, and agitation to accomplish it.
Below is a video of my freshwater skimmer on my small (500 gallon) koi pond.
koi pond fractionation
you can see, my bio-fractionator is a DIY type, which cost me under $40 to build with parts from the hardware store.
The pump is a 2400 gph model