When Georg Smit and Alf Nilsen introduced live rock to American hobbyists in the 1980’s the keeping of saltwater animals changed forever. Up until this time, undergravel filters were the main means for biological filtration and most tanks were decorated with dead bleached coral skeletons. Up until this time, this was the standard method for keeping a saltwater tank.
When these pioneers explained what live rock was and how it was integral and superior to the keeping of marine animals, everything about the hobby changed. When starting out it is difficult to understand how rock can be alive. But marine live rock is not like terrestrial rock in that it forms as the result of corals dying and their remaining calcium carbonate or aragonite skeletons being filled with everything from bacteria, encrusting algae and sponges to microfauna and worms.
Unlike terrestrial rock that contains no space, it is open and filled with spaces that are colonized by everything that lives in the sea. It is so open and full of spaces that it has been estimated that the surface area of live rock is greater than that of the gravel in undergravel filters or the bio balls in trickle filters. That is why it has become the standard for use and critical for filtration in modern reef aquaria. To put it simply, it is the foundation upon which reefs and our hobby reef tanks are built.
Living reef rock
Up until relatively recently, “live rock” was actually live. That is, it was chunks of old dead coral that had broken away from the reef and were taken from various reefs around the world and used as the basis for our reef tanks. Sadly, due to its composition, it has also been used as a gravel substitute in roads after being ground up, and in concrete for buildings, by individuals who had little regard for its value.
As a result of so much of it being taken its removal has now been banned by many countries, so substitutes have now replaced it. When I started in the hobby, my first live rock came from Hawaii and was light and porous owing to some of it being coral skeletons and some of it being old volcanic rock. Removal of this rock was soon stopped in the late 1980’s. Fortunately, live rock was still available then from Florida, Indonesia, Fiji, and Tonga.
Live rock from Florida became such a lucrative business that the first live rock I purchased from there was hand-delivered to me in a bus that was outfitted with tanks and had circulation and air stones keeping the rock alive while the driver drove up the coast from Florida and delivered the live rock to my door.
The taking of live rock from Florida stopped over twenty years ago and live rock from other countries stopped being available several years ago. Real live rock from Florida was replaced with maricultured rock. This maricultured rock is rock that is land-based that is dumped into areas near reefs where it takes on some of the characteristics of “live rock” in that bacteria and benthic organisms colonize it.
The biological function of live rock
Live rock performs several critical and beneficial functions in a reef tank, the most significant of which is to act as the primary mechanism for biological filtration. Since most of our tanks are closed systems full of fish and corals they need to be fed. This food is either consumed, broken down, and released in some form releasing ammonia, or unconsumed and decomposed.
Either way, these waste products need to be broken down and the organisms in and on the live rock facilitate this. If there are filter feeders and other consumers on the rock they will act to consume this waste and break it down into smaller particles. These smaller particles will then break down into the undesired compounds of ammonia, nitrite, and phosphate. The bacteria in the rock then converts these compounds into the less toxic compound of nitrate.
This nitrate is then assimilated by algae and anaerobic or anoxic bacteria in the rock, which convert this nitrate into nitrogen gas which then leaves the aquarium. It should be noted that while ammonia is toxic to fish it is the preferred form of nitrogen for corals. So in the early stages of a reef tank while the corals are small the live rock will be the primary consumer of ammonia.
Over time as the corals grow they will consume more ammonia and the live rock will act less and less to remove ammonia. Live rock is more efficient than any mechanical or external filter at removing waste owing to its huge surface area covered in bacteria. It is also more efficient at reducing nitrate owing to it containing anaerobic and anoxic spaces within the rock, which for the most part does not occur in other filtration devices.
Live rock from different locations in and around the reef, as well as from different locations around the world, contains a wide variety of life, and is the best way to introduce a diverse array of bacteria and other microorganisms and microfauna into a reef tank. In addition to beneficial bacteria, live rock can also help to introduce sponges, which I have found to be of critical importance, coralline algae, filter feeders, amphipods, copepods, worms, and other microfauna into a closed system.
All of these organisms contribute to establishing a healthy system and to the overall water quality. That is why starting with good quality live rock has proven to be so beneficial to establishing a successful reef tank. Because of all the benefits that true live rock provides, if possible, I would always try to start a new tank with it.
Unfortunately, due to availability, cost and other factors starting a tank with true live rock is not always feasible. As a result, maricultured rock as well as synthetic or manufactured dry rock is now being used by more and more hobbyists as a “live” or wet live rock substitute. This rock now comes in a wide variety of shapes, sizes, and colors and for some, it has proved to be at least a reasonable facsimile.
Dry live rock is available in a wide variety of materials including ceramics, concrete, and even resins. Another type of dry rock is rocks taken from the ground that are actually quarried. Some of these are mined from ancient coral reefs and are just the old limestone skeletons from these reefs. Some of these dry rocks include Marco rocks, South Sea shapes, and dry base rock to name just a few types that are available. These dry rocks are available in a wide variety of shapes and sizes including slabs, pillars, and tables with some even containing the tiny nooks and crannies that are so desirable in live rock.
As with most things in this hobby, there are pros and cons of using this rock from my experience. The biggest pro is that this rock is totally free of pests, unlike true live rock. So when it is used there is no risk of introducing unwanted hitchhikers like vermetid snails, aiptasia or majano anemones, predatory bristleworms, or any of the other myriad of pests that have become prevalent in our tanks. And since it is mined or made it does not require any removal of live rock from the reefs. Which may be environmentally better. And, since it is dry, the structure that will be in the tank can be built before it is placed in the tank.
This includes cementing or epoxying pieces together and allowing them to cure before adding them to the tank. Also, since it is dry, the desired pieces can even be selected rather than hoping that pieces will fit together, as is the case with wet live rock.
In my own experience, being able to construct a full 3D reef structure outside of the tank and without having to work underwater is the most positive aspect of using this rock. But as mentioned above there are also cons to using this rock. While pests are not added when using this rock, using it also precludes adding the wide diversity of life, especially bacteria that comes when live rock is used. Also, in my experience, this rock requires significantly longer to “cure” than does live rock. This will be discussed in my next article.
Lastly, and this is my opinion, this rock seems denser and “bulkier” than wet live rock, and for me at least, was initially more difficult to work with. This may have been the result of my not having worked with it as much as wet live rock, but it just did not seem to produce as natural of a structure as wet live rock.
The use of live rock has been one of the fundamental aspects of reefkeeping since it was introduced almost forty years ago. Over time its use has evolved from exclusively taking old live rock from the reef to quarrying it from land-based sites. It is now possible to have successful tanks with either type of rock, as long as their differences are managed properly. Regardless of which is chosen, the goal is to have the rock aid in biological filtration as well as provide a structure upon which corals and other organisms thrive.