From what I have seen your Skimmer needs to have a layer of protein in the neck in order to properly draw up the foam it produces into the collection cup, I clean mine once a week or when I see the skim mate at a certain level in the cup. The gunk build up is certainly out of the tank and no longer in the water. The problem I have seen (and this is first hand) is when you allow some of that gunk build up to fall back in to the skimmer, you start running into problems. For example I cleaned my skimmer some time back last week, and when I placed everything back I adjusted the foam to be a little higher than normal not much but a hair past where the collection cup neck and the body meet at that o-ring. After a few days everything was fine and then one night when I got home from class late in the evening I heard splashing water which we all know that is not good. I opened the door and found the skimmer had been overflowing and not really skimming, I later found out thru a friend, by setting a skimmer a little higher than normal some of the protein gunk falls back in to skimmer and causes the skimmer to go into overdrive. So long story short you need that small layer of protein gunk in the neck but not a lot just enough to help push the foam into the collection cup.