Been there, done that, wasted my money if I was expecting what the packaging said. Purigen can be good filter media, but it isn't going to do a thing for surface proteins.
Let me see if I can explain why/how enough for you to understand.
When we add things to the tank, such as food... many of these things have oil bases in them to help them bond, or heavy protein base to them, which, once it hits the water, leeches into the water. Because of the molecular structure of these proteins, they then float at the surface causing what looks like an oil slick. Purigen and other such chemicals go into your filter. Water must pass through them for them to clean it. Your intake on your filter is down low in the tank, which means it isn't going to suck anything from the surface. Thus, the proteins at the surface are still there, collecting and preventing oxygen and other gas exchanges from being possible. Left alone for any amount of time and these surface proteins will suffocate the tank beneath it.
When using a skimmer, they are designed to draw water from the surface, and using air forces the proteins to seperate from the water. The proteins are then collected in the skimmer cup while water is pushed back into the tank.
One way to avoid surface protein buildup in freshwater is to add circulation at the surface, typically an air stone. This isn't such a good idea in saltwater because of salt residue, otherwise known as salt creep. If you get the fine mist on the surface from an air stone or other heavy surface circulation, that water spray will dry on your equip, your tank, and anything else it comes into contact with, leaving behind a residue of salt, which will damage furniture, carpteting, etc.
Does this make more sense for you?