I was having a probelm with a skim on the surface of the water. I'm using a bak pak skimmer on a 40 gal. tank that does have alot of bio load. I was running carbon 24/7 in the fuval 303. My lfs suggested I replace the carbon with new carbon on my next water change and run that for about 3 days and remove it which I did. Well, the skim is gone. and the water is still nice and clear. I plan on doing this once a month.I have be doing my water changes once a week. Hope this is helpful. It seems to work for me.