Normally protien skimmers are useless on freshwater tanks because of the speciffic gravity of the water. However I HAVE used a modifyed Skelter to good effect on a heavilly stocked freshwater tank in the past.
What I did was to glue a solid acryllic 1.5" rod to the center of the reaction chamber and then replace the rather weak venturi driven bubble generator with a powerful needle wheel type. This allowed the serface tention of the bubbles to lift debris up the narrower chamber and into the collection cup. In other words I basicly increased the power with the needle wheel pump and decreased the area of the reaction chamber.....overdriving the unit.
It took about 2 days for the levels to build up in the reaction chamber but after that I was dumping the collection cup every other day.
I ran this tank that way for over 4 years before I replaced it with saner stocking levels.
I should also note that this tank ran a salinity level fairly close to that of a brackish tank.
I also ran a larger, powered, skimmer on a 12,000 gallon pond with mixed results...it DID in fact work....to a degree but it proved to be more of a headache that it was worth to keep running. I abandoned the project after a few months.