A protein skimmer would be very tricky to get to work in FW
The main problem lies in the water, SW
bubbles easily, and thus the foam is able to be maintained to push it over the collection cup. FW
bubbles do not last long, and thus maintaining the bubble wall to extract DOCs is very difficult.
Try it for yourself - grab a bit of airline, and a tub of water. Now blow bubbles into the water, and note how long they last, next add a few teaspoons of salt (can be table salt as you're not adding to a tank), and repeat the process....
That's kind of the fundamentals of protein skimming.
there is a surface skimmer for the top surface this may be what the other people are referring to.