I have my first skimmer and I've been emptying it out way before the collection cup is full. I've been reading that the slimy stuff that accumulates on the walls of the tube where the bubbles are actually helps the bubbles climb and dump the skimmate in the cup. Does the skimmer work better if this slimy stuff has had a few days to build up? If emptying the collection cup isn't an immediate need, how often should you clean it any way?