Protein skimming also works in fresh, it just takes more agitation at the air water interface to fractionate.
It happens naturally, whenever you see foam from wave action, or near water falls, that is fractionation.
I have used it for years on fresh water tanks, and in my ponds, and feel (when working properly) is one of the most effective filtration methods out there, as it takes waste out of the water column, as opposed to simply storing it.
click to run video of the DIY version working on my koi pond.