Skimmer or Refugium

[Lasse]

This also explains why things like Melafix (containing oils in a water-soluble form) and soap are removed by protein skimmers.

They are not removed in a concentrated form in the foam -they cause the foam to collapses (the concentration mechanism) and many times make the skimmer overflow and the concentration in this overflow out in the skimmer cup is the same as in the water column. It is a type of WC.

Freshwater skimming will not concentrate unwanted compounds very well but for brackish water it works sometimes even for some nonpolar compounds. Skimming in saltwater above 25-30 PSU only works for amphiphilic substances (read mainly proteins) - but on the other hand it works very well.

protein skimmer to produce more foam are not necessarily removed by them

The foam consist of concentrated substances - proteins in water above 25 - 30 PSU are one example. But those substances you mentioned does not produce more foam (IME) in this type of saltwater - they make the foam to collapse - which means concentrations not much above the concentration in the water column.

Sincerely Lasse