I have actually asked myself that a few times(why didn't I drain). I probably shouldn't call it sludge. It seems to be mostly algae, dirt, bugs and after adding the clarifier some of it has clumped together. Turned the pump off last night and let it settle. This am noticed a few spots where the water is clear and you can see the bottom. There was kind of a horseshoe shape where you could see the dirt and even in those spots the water was still pretty cLear for a ways down. We vacuumed up quite a bit(had to have two people and even then its rough). Going to let it settle and try again