Skimming off proteins works only dependent on the amount of minerals in the water. The brink for a functioning protein skimmer is around 2% salt contents in the water. Dissolved minerals (ions) are covering the surface of an air bubble generated either by air stones, venturis or needlewheels. Ions have a positive or negative electric charge and attract the huge protein molecules (and other dissolved organic matter, DOC) and keep them locked while they are fusing to form protein foam which is called flotate. Freshwater skimming with nearly no minerals dissolved works only if completely different constructions are in place. A skimmer made for the purpose of seawater however cannot work with freshwater.