For a skimmer to function maximally, the following things must take place:
1. A large amount of air/water interface must be generated.
2. Organic molecules must be driven to and/or allowed to collect at the air/water interface.
3. The bubbles forming this air/water interface must come together to form a foam.
4. The water in the foam must drain without the bubbles popping prematurely.
5. The drained foam must be separated from the bulk water, collected, and discarded.
Protein Skimming: How It Works
We've just discussed a couple of things that cause problems with number 4 in both directions.....stopping proper bubble formation and causing too many bubbles.
Proper bubble formation is important and I think this is where this topic needs to head. The bubbles typically keep collecting in the top of the riser tube and the water falls back down and goes back to the tank.
A lot of non-organic items are not bound to the bubbles by chemical reaction and those go back to the tank. Also heavier items like detritus are going to fall back down the riser tube and head back to the tank.
Wouldn't running a wet skimmate allow for mechanical filtration as well as chemical filtration? IMO, this is the best mechanical filtration available because there isn't a filter pad collecting rotting detritus that will raise phosphates and Nitrates.