I"ve been getting PMs regarding live rock, so I thought I'd post this to describe its functions, what it will and won't do, and the different 'levels' of what LIVE really means to us in the hobby. Since rock does affect the well being of marine life, especially fish, I thought this might be an apppropriate subject to cover in this Forum, as it relates to fish care.
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 soil, in water, on human skin, etc. In the marine aquarium, they attach to any surface that will have them (the aquarium wall, substrate particles, equipment surface, pipes, filter media (floss, cloth, carbon, resins, etc.), decorations and of course, live; dead; 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. Keep reading.
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.
NOTE: The terms I use are not universally agreed upon. In fact if you go to your LFS and ask for 'LIVE rock' you might get any kind of rock. I mean by this that the LFS doesn't have to meet any standard on what LIVE means. So, I sort it out by giving different names to the different levels of what LIVE means -- my own kind of names and definitions. Don't get hung up on the words I've chosen, but do focus on the quality of the rock being described!
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.
A ROSE BY ANY NAME. . .
There is no national or international standard when it comes to naming marine aquarium rock. Even if you don't use my naming system (see below) 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. LIVE rock is expensive, so be sure you know what it is you're paying for. 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? Have any of those lifeforms died and will require further curing? If so, find out what they are. If you're told it is 'live rock' because it has coralline growing on it, don't pay a premium price for 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. That is why I've built in the category/name of dead rock and base rock to account for this fact.
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 -- 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.
Don't forget how I began this post. The nitrifying bacteria just need a place to attach to and do their job. ANY surface area will do. So all rock will be a place for nitrifying bacteria to live. ALL rock offers surface area. But there is that other set of bacteria I mentioned and haven't discussed yet.
THE VALUE OF LIVE ROCK
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 observe and can measure: 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.
THE NEW KID ON THE BLOCK -- CULTIVATED LIVE ROCK
This is what I'm calling aquacultured live rock (see above). Artificial rock (made of carbonates) and dead rock harvested from the oceans can be placed in areas of the ocean where the rock can begin to take on the organisms that make the rock LIVE. Unfortunately, the process takes years or decades, depending upon just how LIVE the rock must be.
There is nothing wrong with using this rock. In fact, it does help the environment by reducing the harvesting of really old rock. It's value depends on how long it has been in the ocean and whether or not it was in the 'right place' in the ocean. This topic is too long to get into here, but know what it is you're buying. You want to know where it was cultivated (you want it from tropical waters if your aquarium is tropical), and you want it in the right place for no less than 8 years, 15 being the preferable.
This leads us to another thing most misunderstood by hobbyists -- can base or dead rock turn into LIVE rock in the 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. 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 that make homes for the denitrifying bacteria inside the true live rock don't reproduce in our aquariums These worms and other special lifeforms responsible for the rock giving homes for denitrification bacteria, 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.
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 pounds. 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 percent 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 (FOWLR) 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, and another post!
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 a FOWLR 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 gallons/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. Make sure you read this post: http://www.reefsanctuary.com/forums/fish-diseases-treatments/34037-what-water-quality.html
Will dead and base rock also harbor the bacteria that perform denitrification (nitrates converted to nitrogen gas and other chemicals)? The answer is, "YES." However, it doesn't reach any kind of high beneficial level. Also, live rock seems to have a positive impact on marine aquariums beyond just these bacteria. The scientists are still trying to figure it out. Remember these denitrifying bacteria are also everywhere, but they only thrive where oxygen is low. So if the rock is pourous, some dentirifying bacteria will survive there, maybe even thrive to some extent. BUT this isn't enough to control nitrates in a reef aquarium.
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).
LASTLY -- WHAT'S NECESSARY
If you're building a marine system just for fish and some clean up crew invertebrates, live rock isn't needed. Any decoration or rock suitable for marine life forms can be used to create surface area for the nitrification bacteria. You don't need the denitrification bacteria because these marine organisms (fish and most clean up crew lifeforms) are not sensitive to high levels of nitrates. However, a reef tank containing immobile invertebrates is another thing.
A reef tank with immobile invertebrates needs either live rock or a means to remove nitrates from the water as soon as they are made by the nitrification bacteria, or a combination of these. This can be LIVE rock or a chemical filter or huge and frequent water changes (which we tend to avoid).
Finally, live rock looks good! It brings with it all sorts of interesting creatures which adds to the marine aquarium experience. It also provides a positive yet somewhat mysterious advantage to a successful marine aquarium. These are very good additional reasons to use it. It also does give a type of natural look to the landscape -- but so does artificial rock!
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I hope this post has clarified and enlightened the reader on live rock. All rock has its place. Just be sure you know what you're buying, buy what you want it to do, and buy enough!
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 soil, in water, on human skin, etc. In the marine aquarium, they attach to any surface that will have them (the aquarium wall, substrate particles, equipment surface, pipes, filter media (floss, cloth, carbon, resins, etc.), decorations and of course, live; dead; 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. Keep reading.
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.
NOTE: The terms I use are not universally agreed upon. In fact if you go to your LFS and ask for 'LIVE rock' you might get any kind of rock. I mean by this that the LFS doesn't have to meet any standard on what LIVE means. So, I sort it out by giving different names to the different levels of what LIVE means -- my own kind of names and definitions. Don't get hung up on the words I've chosen, but do focus on the quality of the rock being described!
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.A ROSE BY ANY NAME. . .
There is no national or international standard when it comes to naming marine aquarium rock. Even if you don't use my naming system (see below) 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. LIVE rock is expensive, so be sure you know what it is you're paying for. 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? Have any of those lifeforms died and will require further curing? If so, find out what they are. If you're told it is 'live rock' because it has coralline growing on it, don't pay a premium price for 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. That is why I've built in the category/name of dead rock and base rock to account for this fact.
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 -- 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.
Don't forget how I began this post. The nitrifying bacteria just need a place to attach to and do their job. ANY surface area will do. So all rock will be a place for nitrifying bacteria to live. ALL rock offers surface area. But there is that other set of bacteria I mentioned and haven't discussed yet.
THE VALUE OF LIVE ROCK
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 observe and can measure: 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.
THE NEW KID ON THE BLOCK -- CULTIVATED LIVE ROCK
This is what I'm calling aquacultured live rock (see above). Artificial rock (made of carbonates) and dead rock harvested from the oceans can be placed in areas of the ocean where the rock can begin to take on the organisms that make the rock LIVE. Unfortunately, the process takes years or decades, depending upon just how LIVE the rock must be.
There is nothing wrong with using this rock. In fact, it does help the environment by reducing the harvesting of really old rock. It's value depends on how long it has been in the ocean and whether or not it was in the 'right place' in the ocean. This topic is too long to get into here, but know what it is you're buying. You want to know where it was cultivated (you want it from tropical waters if your aquarium is tropical), and you want it in the right place for no less than 8 years, 15 being the preferable.
This leads us to another thing most misunderstood by hobbyists -- can base or dead rock turn into LIVE rock in the 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. 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 that make homes for the denitrifying bacteria inside the true live rock don't reproduce in our aquariums
These worms and other special lifeforms responsible for the rock giving homes for denitrification bacteria, 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.
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 pounds. 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 percent 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 (FOWLR) 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, and another post!
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 a FOWLR 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 gallons/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. Make sure you read this post: http://www.reefsanctuary.com/forums/fish-diseases-treatments/34037-what-water-quality.html
Will dead and base rock also harbor the bacteria that perform denitrification (nitrates converted to nitrogen gas and other chemicals)? The answer is, "YES." However, it doesn't reach any kind of high beneficial level. Also, live rock seems to have a positive impact on marine aquariums beyond just these bacteria. The scientists are still trying to figure it out. Remember these denitrifying bacteria are also everywhere, but they only thrive where oxygen is low. So if the rock is pourous, some dentirifying bacteria will survive there, maybe even thrive to some extent. BUT this isn't enough to control nitrates in a reef aquarium.
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).
LASTLY -- WHAT'S NECESSARY
If you're building a marine system just for fish and some clean up crew invertebrates, live rock isn't needed. Any decoration or rock suitable for marine life forms can be used to create surface area for the nitrification bacteria. You don't need the denitrification bacteria because these marine organisms (fish and most clean up crew lifeforms) are not sensitive to high levels of nitrates. However, a reef tank containing immobile invertebrates is another thing.
A reef tank with immobile invertebrates needs either live rock or a means to remove nitrates from the water as soon as they are made by the nitrification bacteria, or a combination of these. This can be LIVE rock or a chemical filter or huge and frequent water changes (which we tend to avoid).
Finally, live rock looks good! It brings with it all sorts of interesting creatures which adds to the marine aquarium experience. It also provides a positive yet somewhat mysterious advantage to a successful marine aquarium. These are very good additional reasons to use it. It also does give a type of natural look to the landscape -- but so does artificial rock!
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I hope this post has clarified and enlightened the reader on live rock. All rock has its place. Just be sure you know what you're buying, buy what you want it to do, and buy enough!
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