I know it's a bit of a dormant thread, but just sharing some of my experiences. I used to clean my sump bi-weekly. I started to slack off on being obsessed with a clean sump (maybe clean every 3-6 weeks). I'm starting to find a trend with algae problems when my sump gets a certain level of detritus settled on the bottom. I don't see any change in my NO3/PO4 tests. I think what is happening is the nutrient increase is slow and algae growth increases to use it, which keeps it low. My best guess is this funk on the bottom isn't in the water column, so my skimmer can't remove it. It slowly decomposes and feeds the algae. I'm currently working on keeping the bottom agitated and not settling. That way it can hopefully be skimmed or captured in daily water changes better.