What Is Live Rock?
It's a misconception that live rock (LR) itself is alive. What makes it "live" are the many forms of micro- and macroscopic marine life that live on and inside of it. The rock itself is only made up of the calcium carbonate skeletons of long dead corals or other calcareous organisms.
There are different types of live rock. In J. Charles Delbeek's article "Your First Reef Aquarium," published in Aquarium USA in 1994, the Live Rock section refers to "reef rock" basically as pieces of coral or coral rock from outside the reef. The coral has broken off and fallen to the bottom, becoming covered with encrusting organisms, such as coralline algae and sponges. Delbeek refers to "inshore rock" as rock from inside the reef that has a tendency to be denser and becomes covered with macroalgae, clams, mussels, crabs, shrimps, and other unwanted organisms. In Delbeek's view, reef rock is much more desirable than inshore rock because it cycles more quickly and stabilizes a tank much faster.
There is also dead base rock, meaning it has no live growth on it. The rock is devoid of external life that probably won't see much light, so you can put other forms of more advanced live rock and corals on top of it to build your reef system base, once your tank has settled and the base rock is seeded or cured. Beginning a reef tank using seeded base live rock as the center stones of the aquarium is not a bad idea. Once the base rock is established, you can begin to add, slowly, more advanced types of live rock.
Some descriptions of live rock sold by suppliers can be confusing. For example, live rock suppliers describe their Pacific live rock as "actually pieces of coral skeletons that have broken off reefs during storms. This 'rubble' washes in toward shore where it is collected in shallow water." So, whether this a lighter type of "reef" rock or a heavier type of "inshore" rock is unclear. In Live Rock Part 5 - Tips For Buying LR Locally or Online, this topic is covered in greater detail through questions to a supplier about what type of live rock you will be receiving.
What Is the Purpose of Live Rock?
Live rock becomes the main biological nitrification base or biological filter of a saltwater aquarium while, at the same time, enhancing the look of the aquarium and providing shelter for the inhabitants. To better understand what purpose live rock serves in an aquarium, Delbeek explains it like this:
"The use of live rock immediately introduces into the aquarium numerous algae, bacteria, and small invertebrates, all of which contribute to the overall quality of the aquarium water. Live rock has just as much, if not more, surface area for bacteria as a trickle filter. Since live rock in the aquarium contains various types of bacteria, algae, and corals, waste products such as ammonia, nitrate, and phosphate can have a number of fates. Ammonia, nitrate, and phosphate are readily assimilated by algae and photosynthetic corals growing on and in the rock. Ammonia can also be quickly converted into nitrate by the bacteria on and in the rock. This nitrate can be absorbed by the algae and corals, or bacteria can denitrify it in proximity to the nitrate producing bacteria."
No matter what type of live rock you choose to use, as you may know, a biological filter base has to cycle and settle for your aquarium to run properly. This pertains to the curing of live rock as well.