If you have begun to do even a little amount of research into the setup of a marine aquarium (be it reef or fish only) you will undoubtably have come across some mention of live rock (in webspeak you will often see live rock abbreviated to simply: “LR”). The obvious first question which must be answered is “What is live rock?” Contrary to the name, the rock itself is not actually alive, however it is covered with thousands, if not millions, of tiny microorganisms, macroorganisms, and bacteria. The list of organisms frequently imported on or inside live rock is long and distinguished however the most important aspect of this live rock is the beneficial bacteria which is present within every nook and cranny. This bacteria is what converts ammonia in your aquarium into nitrite and that nitrite into nitrate. Coupled with a protein skimmer (especially in larger systems) and appropriate maintenance, live rock will provide all of the necessary biological filtration in a marine system. That means no more bio-balls, bio-wheels, filter pads, filter floss, etc. are necessary. Live rock does have a few special care needs but they are far from difficult to meet. Live rock does require a minimum amount of light in order to maintain the health of some of the photosynthetic organisms which live in and on it. Among those organisms are the coveted encrusting coraline algae and other macroalgaes. For a fish only tank the standard flourescent tubes common to most hoods will provide all of the necessary light for maintaining these organisms; obviously in reef tanks more light will be necessary for maintaining corals. Beyond light, live rock requires water flow in order to prevent the development of dead spots. Powerheads and sump return outlets should be arranged to ensure that all areas of the live rock structure receive water flow. It is important that detritus not be allowed to settle on the live rock; if it is impossible to prevent detritus from accumulating in some areas of the rock structure it is important that a vacuum be used to remove that detritus. Beyond minimal lighting, it is important that live rock be kept in water which is within standard marine aquarium parameters. This means a specific gravity between 1.022 and 1.026 (note there are no units because this is a ratio of two densities). The pH of the tank should be maintained around 8.2 or 8.3. Temperature is also important; live rock should be kept in water between 75 degrees Fahrenheit and 82 degrees Fahrenheit. While many of the inhabitants of live rock will survive at higher and lower specific gravity, pH, and temperatures it is in the best interest of all living organisms on the rock that those ranges be maintained. At the local fish store there should be quite a few different types of rock to choose from, but the two major categories are cured rock and uncured rock. When live rock is imported it is teeming with life, much of which dies during shipping or will die in the holding tank at the fish store. When all of that die off has occurred the rock is then considered “cured” because, in theory, it can be placed in your home aquarium and nothing will die on the rock. In practice, stuff is always dying within the rock so what “cured” really means is that the most significant die off will have occurred and any subsequent die off will not be at a level which will affect the stability of the system. Uncured rock is essentially fresh off the airplane. Generally uncured live rock is cheaper and will often look much more rich in terms of algae growth and organisms, however much off that life will die off. When you buy uncured live rock it must then be cured by you, a process which involves a fair amount of work, but can be worthwhile if you are buying a large volume of rock. A good test for the health of rock is its smell. Rock which is cured will smell like the ocean, but it will smell fresh; uncured rock, or rock which is curing, will smell a bit like rotting seafood. Beyond the distinction of cured and uncured you will find that the fish store often labels some live rock as base rock. Base rock is generally dense rock which is not particularly attractive, but is significantly cheaper by the pound than other types of rock (Fiji, Tonga, Marshall Island, etc.). This rock is called “base” because in large systems people use it to create the foundational structure for their aquascaping. Base rock is, in fact, dead. There is nothing alive on base rock, though stores may sometimes mislead the consumer into believing there is. The idea is that the aquarist can buy a lot of base rock and a few pieces of live rock and allow the organisms to spread. Base rock is not particularly well suited for small systems because it is important to have adequate beneficial bacteria right away. Base rock can take months, and even years, to seed. Something else to consider when buying live rock is that while base may be cheaper by the pounded it is also quite dense and so a piece of “nicer” rock which weighs the same will generally be considerably larger. As alluded to before, the less dense the rock is the better it is for the growth of beneficial bacteria and organisms. More surface area is good even if that surface area is from tiny pores in the rock. When selecting live rock it is important to consider whether it is cured, the price of the rock, the relative density (pick up a couple different pieces), the observable life present, and the structure. Going to a few stores (if possible) and comparing these traits is an important part of securing the best rock for the best price. Some stores may be willing to bargain a little and many stores offer discounts for bulk purchases. The eternal question surrounding live rock may be “How much do I really need?” Live rock is quite expensive and so this is a reasonable question. In all practicality it is, unfortunately, difficult to answer. Many people recommend one pound for every gallon of water; some recommend more and some recommend less. In reality the answer to the question varies depending on the density of the rock, the stocking plans for the tank, the efficiency of the protein skimmer, and the diligence of the aquarist as far as maintenance is concerned. Aesthetics plays an important role in the selection of rock. Generally speaking, aquarists (particularly in the United States) have an excess of live rock in their systems relative to the amount necessary and so the best way to decide how much is enough may come down to what looks best. With dutiful water testing it is easy to determine if more biological filtration is needed; if it is so discovered more rock can easily be added. Live rock requires no acclimation. It should, however, be placed into water at the appropriate parameters as soon as possible. Some people recommend quarantining new live rock to help catch unwanted hitchhikers. It is certainly much simpler to remove a mantis shrimp or unwanted crab when the rock is easily removed from the tank than when it has been integrated into the aquascaping. Once the pest is in the main display tank, particularly large tanks, it is often quite difficult to remove. Remember, also, that if uncured live rock is bought (and even cured rock if it is from an unfamiliar source) it should not be added to an aquarium with animals already present. The die off can cause a significant ammonia spike. It is always better to be safe than sorry. The curing process, while not overly complicated, is a subject best covered in a future article. Concerns are sometimes risen about the environmental impact of the collection of live rock. Live rock is actually not taken from the reef itself, instead it is pieces of the reef which have broken off the reef naturally. Beyond that wild collection there is a great deal of commercially aquacultured live rock available. This rock tends to be aquacultured in the Carribean whereas most collected rock is imported from the Pacific so mixing the two types can help with the bio-diversity of a system. Diversity is always good. In the long run, live rock is the key to a healthy marine system. There is certainly much more to learn about live rock but this provides a good introduction for the new marine aquarist. The internet can be a valuable resource, especially when looking for images of different types of rock. There are also many online vendors from whom large orders of rock (50 pounds or more) will be cheaper than buying locally. It is, however, still important to visit the local fish store, see what they have, and become familiar with the different types of rock up close and personal even if an online order is in the future. Remember, patience is the key to a successful marine aquarium; having a strong theoretical foundation and a good plan will, however, make the process run much more smoothly.