a rather confusing term, "carbonate hardness", or KH. This is usually measured in "degrees" (dKH) or meq/L. The microfauna found on live rock are detrivores and herbivores (as they eat algae and fish waste), and provide fish with a natural, attractive shelter. Live rock usually arrives from online dealers as "uncured", and must be quarantined in a separate tank while undergoing the curing process, which involves the inevitable die-off of some of the rock's inhabitants and the subsequent production of undesirable ammonia and nitrite. Live rock that is already cured is available at most pet stores that cater to saltwater. Live sand is similar to live rock and is equally desirable. Sometimes hobbyists use so-called "dry rock", which is simply old live rock that has been allowed to dry out and to lose most of its live inhabitants, to keep unwanted pests out of their aquariums, and as an inexpensive alternative to live rock. Filtration In general, marine aquariums have more complex filtration requirements than most freshwater aquariums. The various components frequently include wet and dry filters and protein skimmers. Protein skimmers are devices that remove organic compounds prior to their degradation, and are very useful in marine aquariums. Protein skimming is also used in the popular Berlin method that relies on live rock and periodic partial water changes to degrade and remove waste products. The Berlin method requires large amounts of live rock in the aquarium. The rule of thumb is 1/2–1 lb. per 1 US gallon (0.2–0.4 kg per 4 liters). Many marine aquariums include a sump, which is an external container connected to the main aquarium with a water pump. In most setups, the sump is located below the aquarium and is fed water from the main tank via an overflow. An overflow at its simplest is a round hole drilled towards the top of the tank, connected via tubing to an output below it. As the water level increases past the height of the overflow (weir), the water "overflows" the tank and falls through to the sump below. It is typical to have tall combs on the overflow edge so fish do not jump inside. The water circulation is powered by a water pump in the sump, which pushes the water up into the tank, thus causing more water to flow over and perpetuating the cycle. There are many advantages to using a sump, both for