Neil H

Rock and Sand in the Reefers World, A geologists Perspective.

  • Much has been said on this and other marine forums on the subject of rocks and sand in various forms in the reefing hobby…. I thought it may interest some of you to have a more in-depth perspective on this subject. All views expressed are in my opinion and experience. :razz:

    Three major rock types occur on the face of the earth these are igneous rocks, metamorphic rocks and sedimentary rocks. Typical igneous rocks are formed as a result magmatic activity… simply put these rocks are formed by volcanic activity, a typical examples of this type of rock is Granite.


    What you see in this rock is that there are no real gaps between the individual crystals, the crystals are large, indicating a long period of cooling and crystal formation. Why is this important for us as reefers…. Simple, this rock has almost zero porosity, no water will move through the matrix of this rock, no water, no bacteria. Yes corals will attach to this type of rock and bacteria will colonize the surface, but no real useful filtration is created by adding this to your marine system. Igneous rock is generally only useful as base rock to fill up a marine system, nothing more.

    The second class of rock is metamorphic rock, as the name suggests this rock has metamorphosed from one state to another. The original rock may have been igneous or sedimentary in nature; this is for the most part irrelevant. Basically heat or pressure or both are applied to the original rock, the crystal structure changes and a metamorphic rock is formed. Generally one can see a direction, known to geologists as a fabric within the rock, this indicates the direction of pressure or heat during formation

    This particularly good example of a metamorphic rock is gneiss, one can clearly see the pressure came from the upper left and lower right of the picture. From a marine perspective, metamorphic rock has the same uses and disadvantages as igneous rocks.

    Sedimentary rock on the other hand is what we as reefers should be looking for. Sedimentary rock is formed through 2 possible processes. Firstly when a rock is eroded by water of wind or chemical action and the individual particles are then deposited in another area, over time the particles solidify and form a rock. Secondly there may be chemical precipitation when an environment changes for example an oxygen rich environment becomes oxygen poor, resulting in a element going from its soluble state to an insoluble state.

    The important thing is the fact that the gaps or pores between the rock crystals in sedimentary rock are open….. Pores = porous = water can move through the rock and hence the bacteria can occupy an exponentially larger area than an equivalent sized piece of igneous or metamorphic rock. In the picture below, the black spots indicate air, or pores available for bacteria populations and hence filtration.

    Unfortunately not all sedimentary rock is equal in terms of its porosity, the finer the rock crystals, the smaller the pores, the less porosity. In conjunction with porosity is the concept of permeability, which is a measure of the interconnectedness of the pores….. What we as reefers are looking for is a rock with lots of pores and of equal importance how well those pores are connected to one another. Getting just one of these parameters right is not good enough, we need both a high porosity and a high permeability. Should one find a rock that possesses these qualities and seed this with “live rock” (to be discussed shortly) there is no reason why this rock could not be as effective a biological filtration media as traditional “live rock” that we buy from our local pet shops.

    This brings on the logical question, what is “live rock” ? This confused me to no end when I started in marines….. my first reaction was to get out old text books and find out exactly what this stuff was, find a local source in SA and go and collect it myself… what kind of a geologist would I be if I could not??….. Sadly things are not so easy are they! I found no reference to “live rock” in any recognized geology text, yet there it was in black and white on MASA, and many marine books ….. after asking more that one of my colleagues what they knew of “live rock” and suffering untold abuse for suggesting that rock could be living I decided to hell with it buy some and assess it from first principals.

    There is a wide array of “live rock” from Fiji to Tonga and Kenya, from branch rock to boulders….. but nowhere does anyone describe what it is….. what it does is described ad nausea all over the web and I will not cover that in this discussion, suffice to say it is essential in a reef tank. In my opinion what we refer to as live rock is clearly a type of sedimentary rock, part chemical and part physical. You will note in your live rock at home or in the pet shops that the rock contains many different components….. dead SPS and LPS calcified branches, shells, sea sand and even other pieces of rock. These are held together by a matrix of calcium rich material, which is formed either through some sort of precipitation or through some sort of pressure, although it is my suspicion that the former is the real answer. The rock is highly porous and permeable and if collected from the ocean, full of beneficial bacteria.


    This piece of coral fossil is millions of years old, but add a bit of algae, coralline and a few bugs and it looks remarkably like live rock.

    It is my opinion that live rock is the pre-cursor to limestone and ultimately marble (both rocks with an extremely high Calcium content) If you ever wonder through an old church clad in marble, look carefully and you will spot all manner of coral fossils…. The Grahamstown cathedral is one such example.

    Now that we have covered the different types of rock, and live rock, there is one last class of rocks in the marine hobby to consider, these are the so called bio rock or fake live rock. These come in many different shape size and forums…. Some of the more natural version is mined limestone (a sedimentary rock with high porosity and permeability) …. Given enough time to seed and build up a proper bacteria population there is no reason why this should not provide some sort of useful bacterial filtration perhaps even on par with traditional live rock. Unfortunately there are no comparative studies that I am aware of to confirm or disprove this theory. Then there are the other rocks called by a multitude of names and including ceramic rock “volcanic” rock, pumice etc etc …. The names are endless, and in my opinion provide great aesthetic value (sometimes) but do very little in the way of helping biological filtration, and in my opinion are not worth the astronomical prices often asked for these rocks.

    Now that the different rock options have been discussed, lets move onto the wonderful world of substrate for our marine tanks. There are 5 basic options in a marine tank; bare bottom, Crushed Coral, Reggies Play Sand, Aragonite, and Sea sand. I will discuss each from a geologists and a hobbyists perspective.

    Bare Bottom
    The advantage of this is that zero detritus in locked up in the substrate and thus minimal nutrient problems. I don’t like this personally as it does not look natural in my opinion, also I have some critters who need a substrate so it is not suitable for my reef.

    Sea Sand
    A good option for the DSB (if collected correctly) but less so for the Display tank. Sea Sand is extremely fine, and with the water flow demanded by many corals, a sand storm would be the only result. It also does not have the crisp look that many of us strive for.

    Crushed Coral
    Often one of the first things pet shops try to sell us and almost always with some degree of success, it seems natural to have crushed coral as a substrate to your future reef. In my opinion this is the least desirable of all the substrates… it does not hold its white color for long. It very quickly becomes a trap for detritus, resulting in a nutrient factory and problems with algae such as green hair algae. I cannot discourage the use of this substance as a substrate strongly enough.

    Reggies Play Sand
    This is a cheap way of attaining a good looking substrate… make sure it is washed properly and it will give you a good white look. It is not likely to form a detritus trap and with the particle sizes being larger than sea sand, the likelihood of a sand storm is greatly reduced.

    Often pushed as the ultimate substrate, it has many good points, however its price puts it out of the reach of the average reefer. Aragonite has an SG or density higher than that of Reggies play sand (which is mostly silica) which means that it is heavier per given volume than any of the other substrates and will not be blow around by the pumps very easily. The particles of aragonite are also likely to be rounder than the play sand which is also beneficial to the bacteria (although not essential). Aragonite is bright white and holds this color for a long time. Many people push aragonite as the best substrate because of its slow release of calcium and buffering capabilities (it is after all almost pure calcium) this is simply hogwash in my opinion…. In order for aragonite the release calcium the ph of the system would have to be so low as to kill most if not all marine creatures in the tank. Simply put the calcium requirements of the biological organisms in the tank cannot be met by a geological process. That said if you can afford it, go for aragonite, you will not regret it.

    That’s my little opinion of rocks and substrates in the marine hobby, it is by no means an exhaustive discussion but may shed some light on the subject for newer members of the hobby.:thumbup: