I have a 9010 that I am using in the sump. Generally, it works quite well; however, I have noticed that the amount of skimmate collected steadily decreases over time. The strange thing is that if I take apart and clean the skimmer and pump, it will again produce much more skimmate. It seems that it needs to be cleaned in such a manner about every three to four weeks. Is this normal for this skimmer?