Over long periods of time, detritus will build up in the rock work, under corals, and in any places where the water flow is insufficient enough to keep it suspended in the water column. Despite using multiple water pumps and powerheads to create tons of water flow, dead spots will occur and the detritus accumulation is nearly unavoidable. This problem is especially true for systems where the water flow patterns are unchanging, e.g. they use powerheads that point in a fixed direction. To prevent the dead spots and get the most of out your pumps, try using devices that rotate pump outlets and powerheads. These devices, such as a Sea Swirl, rotate the powerheads and help create the most random water flow patterns possible.