industrious DIYers, but these materials are typically reserved for the construction of larger tanks. Filtration The primary biological filtration for reef aquariums usually comes from the use of live rock which come from various tropical zones around existing reefs, or more recently aquacultured rock from Florida. Some reefkeepers also use what is called deep sand beds (DSB). These are often employed to augment the biological filtration by aiding in the reduction of nitrate, a waste product in an incomplete nitrogen cycle. Deep sand bed opponents may prefer a "bare bottom" or "suspended reef" which allows for easier removal of nitrate-generating accumulated detritus. This biological filtration is usually supplemented by protein skimmers. Protein skimmers use the foam fractionation process wherein air is introduced into a water stream creating microbubbles. Organic waste adheres to the surface of these microbubbles and is removed as it overflows at the reactor surface into a removable cup. This group of elements used in conjunction is characteristic of the Berlin Method, named for the city in which it was first devised. In recent years the Berlin Method is often supplemented with a refugium. A refugium provides many benefits, which include nitrate reduction, as well as providing a natural food source. It typically houses two main species of macroalgae, including Caulerpa prolifera or chaetomorphae or both (because these two strains are known not to spore but grow by rooting to propagate). Macroalgae is used for two reasons: to remove from the water excess nutrients such as nitrate, phosphate, and iron, and to support beneficial microflora and fauna (zooplankton). Small invertebrates (copepods and amphipods) are provided a space free of predation to grow and, when returned to the display tank, serve as food for corals and fish. Conventional combined mechanical/biological filtration used in fish only systems is avoided because those filters trap detritus and produce nitrates which may stunt or even kill many delicate corals. Chemical filtration in the form of activated carbon is used when needed to remove discoloration of the water, or to remove dissolved matter (organic or otherwise) to help purify the water in the reef system. Water movement Water movement is important in the reef aquarium with different types of coral requiring different flow rates. At present, many hobbyists advocate a water turnover rate of 10x: 10 x aquarium capacity in gallons = required flow in gallons per hour⁠—⁠this is mathematically equivalent to a complete