Protein skimmers are not effective for fresh water.
This is not entirely true.
To OP: their is currently an ongoing debate as to whether protein skimming works or not in freshwater. It is a very heated topic, much like the fx5 vs 2262 or the nls vs omega1. Their has been much research on this topic, I know somebody that runs a regular protein Skimmer in full fresh and he gets about a half cup of the protein stuff a week. This proves its effectiveness. But needless to say you cannot just plainly say 'it doesn't or wont work' because such proof does not exist to prove either side. Hope this helps.