The sock sump is used as a strictly mechanical filter, not biological, and must be thoroughly cleaned every few days. Every time you clean the sock you remove organic matter that would ultimately raise nitrates and phosphates if left in the tank. Waste water from the tank is brought down to the sump where it passes through the “socks” as they collect the heavier solids (excess fish food, waste, debris). The filtered water is then returned back to the tank via a pump. This system relies on the bacteria to be colonized in the live rock and substrate in the main display tank for biological filtration. These filtration systems are recommended for coral reef tanks as they give you a safe, and out of the view place, to keep unsightly equipment such as heaters, reactors, and even skimmers which we will explain next.