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One of the important features of the aquarium for fish is how their nitrogen wastes are handled. In an aquarium full of corals, very little nitrogen waste is produced and the aquarist can almost ignore these kinds of wastes. But a marine aquarium stocked with fish, snails, crabs, shrimp, etc., has a large source of nitrogen wastes that need to be processed quickly in order to prevent harm to the fish and marine life.

The aquarist depends upon the microbes in the aquarium to enjoy the ability to maintain marine fishes. In essence, I like to think that the real hobby is how to keep and maintain microbial life!  

The biological filter is the scene of the 'action' when it comes to processing nitrogen wastes (nitrification = converting ammonia to nitrite and nitrite to nitrate), but the bacteria that process wastes are everywhere in (and around) the aquarium. This should be made clear. These bacteria are everywhere. They are found in the air, in sand, in water, on human skin, etc. In the marine aquarium, they attach to any surface that will have them (the aquarium wall, substrate, equipment surface, pipes, filter media (floss, cloth, carbon, resins, etc.), decorations and of course, live or base rock). BUT (and this is important) the real value of real live rock is not as a surface area for these kinds of nitrifying bacteria, but as a home to another set of bacteria.

Live rock has some special characteristics that make it of particular value with regards to its role in handling nitrogen wastes. The aquarist that wants to keep fish and/or a reef aquarium with fish, needs to understand more about this value. First though, let's get on the same page for our names and definitions. There are different levels of life on the rock we use in the marine aquarium.

Rock doesn't live. So how can it be alive?  It's the organisms on and in the live rock that gives it that name. There are different levels of life on/in rock that qualify the value of the live rock.

NAMING ROCK
Total Aquarium Definitions (live rock=straight from the ocean, base rock=live rock that has died or been in one or multiple aquariums and can be used as media in the tank and filter to absorb certain biological bacteria, cured rock=moist transported live rock, submerged in a high bacteria rich environment)

There is no national or international standard when it comes to naming marine aquarium rock. Even if you don't buy into my naming system keep in mind that when you communicate with someone regarding aquarium rock that the both of you are using words that you first define to each other. There are many people in the hobby that would mis-represent rock by carelessly tossing about words. Don't be fooled. Ask.

Rock with no life in or on it is called 'dead rock.' Some people call this 'base rock' but I don't. You'll find that the use of the phrase 'base rock' means different things to different people. Here I want to be clear and, whatever you choose to call it, make sure you and the person(s) you are communicating with are using the same definitions not words.  

Rock with some coralline life on it, but nothing much else is called 'base rock' or maybe you're more comfortable calling it 'live dead rock?' It is more than dead rock, but isn't fully functional like live rock, as it relates to the handling of nitrogen wastes.

'Up' from the level of base rock, rock can have other organisms living on it (e.g., polyps, complex algae, pods, corals, etc.); or organisms living on and in it. This is called 'live rock.' But there are different levels of life on and in live rock, which make live rock have different 'levels' of being alive.

HOW ALIVE IS LIVE?

Rock right out of the ocean/sea is true live rock and stands as the 'most alive' of the live rock. It has this distinction by virtue of the diversity of life on and in the rock. It has a grand diversity of life and lifeforms. It took decades to hundreds of years for that life to take up residence on and in that rock. But this rock can't be shipped to the user, without being submerged in water. Decades ago we used to get this kind of live rock from the sea. It was shipped in water and had ALL the lifeforms represented on and in it as if it was still in the ocean. Today, shipping rates and fuel costs prohibit this. So the rock ships without water. Often it is kept moist by wrapping it in paper/newspaper wet with saltwater.

To ship moist like this without 'rotting' along the way, the rock first has to be cleaned of organisms that would die when they are exposed to air (e.g., sponges). During this process, most (if not all) crabs, shrimp, etc. are also cleaned off.

Many but not all things in and on the rock die during moist transport. So the wholesaler/LFS or aquarist gets the harvested rock and has to further cure it. After curing, this is about as alive the live rock will ever be for the aquarist. It's a considerable difference from the rock that was in the sea as far as diversity of lifeforms is concerned. But this is what we call and mean by 'live rock.'

But, even more cleaning and curing can be done along the way which further reduces the diversity of life on and in the rock so that the aquarist gets various levels of life on and in the live rock. Although curing makes the rock less polluting for the aquarium, it may also destroy and may remove more of the natural lifeforms in and on the rock.

Ever read or hear of fully cured live rock? Seems like an oxymoron. If the rock is fully cured, then it should be dead rock, right? A misnomer for sure, but we accept it. In general, this term means the live rock is ready to be put into the aquarium, but the aquarist should ask just how cured it is. Is there any lifeforms still in and on the live rock? If so, find out what they are. If you're told it is 'live rock' because it has coralline growing on it, don't buy it. Offer to buy it as base rock until other life forms on and in the rock are known to be present.

Just because the rock is covered with coralline, it doesn't mean it is live rock.

There is also 'aquacultured live rock.' This is dead, base, or man-made rock that is placed in the ocean. It will develop lifeforms on and in it, but because it hasn't been in the ocean for decades, the diversity of life on and inside this rock is limited. This doesn't mean this rock is 'bad.' It is very responsible for aquarists to try and seek out buying cultured live rock, in order to spare our natural reefs. But just don't expect the entire width and breadth of lifeforms in such rock as you would from live rock having been in the sea for decades. This rock is also usually shipped moist (wrapped in paper), so some initial or at least final curing is needed.

THE VALUE OF LIVE ROCK

Remember the nitrifying bacteria I mentioned early on that handle nitrogen wastes in the nitrification process (converting ammonia to nitrite and nitrite to nitrate)? These bacteria are on live rock. They are on dead rock and base rock too. The rock just serves as a surface for them to stick to. Fish wastes and oxygen in the water provide their nutrients/food. So no matter what rock is put into the marine aquarium, it will act as this nitrification part of the biological filtration system (after enough of these bacteria multiply). This is not the real value of live rock, however, when it comes to bacteria.

There is another set of bacteria that is responsible for turning nitrates into nitrogen gas (and other compounds). The process is 'denitrification' and is performed by denitrifying bacteria that don't like to live in areas where there is oxygen. They are found in tight-packed soils, in lower levels of substrates, inside of porous rocks and places (like tiny crevices and worm holes (with or without the living worm still there!) where there is low oxygen content.

The real value of live rock in the marine aquarium is what these denitrifying bacteria are doing just under the surface and in crevices of the rock. Denitrifying bacteria (unlike the nitrifying bacteria) can only live and grow where there is little or no oxygen, so we don't find them growing everywhere. (NOTE: They can be found almost everywhere, but just not growing.) The real value of live rock in the biological filtration process is the denitrification it can perform in the marine aquarium.

How live rock performs the denitrification isn't fully understood. Many theories and models have been proposed. That topic is a whole other post in of itself! Scientists try to explain what we know: Live rock not only will function in the nitrification process but will also function in the denitrification process. So the 'right kind' of live rock can help remove nitrates in an aquarium.

GREAT EXPECTATIONS

If the aquarist expects live rock to do the full function of nitrification and denitrification then the rock must be alive in the sense that there are still living organisms (e.g., worms, etc.) inside of the rock AND that the rock will perform denitrification. Such live rock has not been stripped of life. nor cured to kill what's inside (e.g., by freezing, boiling, baking, or chemically treated), nor transported dry (but wrapped in at least moist paper).

Probably the biggest error for new aquarists is the thought that if dead, home made, or base rock is placed next to live rock in the marine aquarium, that these will turn into live rock. IT WILL NOT.

No matter how long dead, home made, or base rock sits next to live rock in our aquarium, the lifeforms that live inside of live rock will not move over to the dead or base rock. Those rocks will never become live rocks! Why? Because the organisms inside the true live rock don't reproduce in our aquariums  These worms and other special lifeforms responsible for the rock performing denitrification, will not spread. They cannot move to other rock and they cannot reproduce/multiply in the aquarium. In fact, this is one reason why some aquarists claim that live rock should be replaced every 5 to 10 years. The lifeforms inside the rock will die off of their own accord, and the help the live rock gave at removing nitrates will diminish as the rock ages. Some claim that once coralline algae covers over the crevices and these worm holes, that the rock will loose its ability to handle nitrates, too. Makes sense. So maybe don't be in such a rush to cultivate a lot of coralline if you need the denitrification!  

What will happen when dead, home made, and base rock is put next to live rock is the organisms that can spread will do so, to the other rock. Such organisms are coralline algae, other algae, pods, polyps, etc. But the worms and specialized lifeforms don't spread! So the aquarist can use base rock, home made, or dead rock mixed with live rock and still have (eventually) a nicely coralline-coated aquascape, but the dead and base rock will not help much in removing nitrates, but merely act as more media surface area.

When the place that you are thinking about obtaining live rock from tells you they made their own live rock, or that they put base rock next to live rock in their tanks, then you should be well armed with the truth -- they did not make live rock. They made base rock. It is only covered with coralline and a few other organisms and lacking the 'inside' organisms.

HOW MUCH IS NEEDED?

The hobby likes to measure live rock in kilograms. But the weight needed really depends upon the density of the rock and how much the rock is alive. The live rock right from the ocean is most efficient, but as previously written, no longer available to the landlocked hobbyist. So to compensate for the loss of diversity and organisms, more live rock is needed to perform denitrification.

Most general rock of 'average density' (whatever that means) needs to be in a marine aquarium supporting fishes at a rate of about 2 pounds per gallon of display tank volume. This weight of rock can be spread out in the display, sump, and refugium to provide more swimming space in the display tank, but at least 75% of it needs to be in the display at the immediate source of where the majority of nitrogen wastes are produced. The is the quantity of live rock needed for a fish only with live rock aquarium. Although marine fish are not sensitive to nitrates, the use of live rock can reduce the frequency of water changes and produce a much healthier environment for these lifeforms. 30+ years ago, live rock wasn't used in fish only aquariums and the fish did just a well in the hands of advanced aquarists. A reef aquarium can get along with less weight, provided this aquarium is understocked with nitrogen waste producing lifeforms (fish, snails, shrimp, crabs, etc.). An aquarium without fish, crabs, snails, and other nitrogen waste producers can maintain low nitrates with very little (or no?) live rock.

When should it be replaced? When the aquarist starts getting elevated nitrate readings (usually after 8 years) with the usual water changes and maintenance and bio-load. How do you replace live rock? That's another story, not easy nor straightforward. Talk to us!

A JOB WELL DONE

In order for the rock to do the best job it can as far as nitrification and denitrification is for the nitrogen waste products to get to the rock. This is where the aquarium system plays a role. Specifically, it is about circulation. The fundamental minimum circulation for live rock is 8 to 10 system volume turnovers per hour. Above this figure, circulation may be needed (e.g., in reef tanks) for certain sessile invertebrates, corals, etc. that depend upon greater circulation to carry off their wastes and/or to bring nutrients to them. So for an aquarium, without special marine life needing additional circulation, the target is 8 to 10 system volume turnovers per hour. The "8 to 10" is 'calculated' by adding up all the liters/hour flow rates of: powerheads, internal pumps, and external pumps (used for systems with sumps and/or refugium).

Now that the right turnover is achieved, the flow must be evenly distributed over and around the live rock. No dead places. All exposed surfaces of the live need to have some flow around them. Adjust and direct the various flows to achieve this goal.

ALTERNATIVES

There are other means to perform denitrification. A deep sand bed is used for this purpose. Special filters are made to house denitrifying bacteria in low oxygen areas. Special media exists to also provide a place for these bacteria to grow. But nothing is as efficient as live rock, even after it's been cured so much by the time it got into the aquarium.

Then there are also chemical means to remove nitrates (absorbing and reacting compounds).

Talk to us about our live rock and what it can do for your aquarium.