Skimmers need the water composition with marine salt added to be able to hold the bubble structure that removes proteins from the tank.
Think of it this way. The ocean has a natural protein skimmer...waves and tides. That white foam on the beach and tops of waves is what a protein skimmer is attempting to reproduce. The crashing waves create massive amounts of bubbles under the surface of the water, churning it and aerating it, and the bubbles collect proteins in the water, gather together in big masses of fluff, float to the surface, and blow off onto the beaches. It is because of the unique structure of sea water that this is possible.
Freshwater bodies don't have the same chemical structure, the aeration is different (even on lake michigan you rarely see white caps and collected foam) and the surface area of the air and water bubbles is not strong enough to hold together and collect the protein.
Skimmers don't work on freshwater bodies because there is nothing to strenghthen the bond of the molecules....where as in the ocean and saltwater bodies, there is...salt.