Live rock is not just a decoration for reef aquariums but can be an extremely useful addition to saltwater systems. It is collected from the vicinity of a coral reef and consists mostly of aged coral rubble (branches or masses of reef-building coral skeletons) of varying sizes. It varies in quality depending on location from which it was collected. The surface of the rock is often brilliantly colored with calcareous algae, macroalgae, sponges, and even small corals. Burrowing organisms such as worms and crustaceans create a labyrinth of caves and tunnels through the rock, often giving it a very porous nature. On the exterior of the rock and in the open pores where water circulation makes oxygen available, aerobic nitrifying bacteria can be found in large numbers. These bacteria perform the nitrification that is present in virtually any biological filter. An interesting benefit to live rock is that it also harbors live anaerobic denitrifying bacteria in its deep recesses that break down nitrate molecules.
Live rock is an excellent addition to the sump of a central water system. At times I have relied solely upon it to filter the water in my aquarium. It is best, however, to rely on live rock as an enhanced source of natural filtration. The bottom of the sump can be covered with cured live rock that is void of organisms that require high light levels. Room light is usually enough to keep certain algae species alive, but a fluorescent light can improve the growth of these plants and also the nitrification of wastes. Live rock is not without disadvantages. The burrowing organisms within the rock secrete waste and when packed tightly together drastically reduce flow rates through the rock. Over time nitrogenous wastes can build up in crevices of the rock. Weekly or monthly purging of the rock will keep these wastes from becoming harmful.
Live rock should not be added directly to broodstock aquariums, as many demersal spawners will place their eggs on or within the rock. For species such as dottybacks, whose eggs must be removed for hatching, the rock makes this task extremely difficult. Gobies will find the tiniest of holes to spawn in, and removal of the fish or the spawn becomes a daunting task.
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