Substrate is the sand or gravel that covers the bottom of the tank. Substrate can be cosmetic, a place for critters to burrow and dig through and it can also be an important adjunct to the biological ecosystem within the reef tank. Old school was that a bare bottom tank was important so that all detritus could easily be removed. That is no longer true and you will not find many bare bottom tanks around these days. Today, there are five main types of substrates in use:
Coarse Crushed Coral Gravel
The coarse coral gravel substrate is falling out of favor. The large particle size allows excessive amounts of food to fall into the crevices and this substrate does not provide a very good biological zone. Many tanks are setup with this substrate through lack of knowledge, but it should not be used even though this is sometimes the only substrate sold at the LFS. Coarse gravel CAN be used in the reef tank if it is used in combination with finer sand.
1.5"- 3" Medium Grain Substrate
The 1.5"- 3" deep, moderate particle size (1-2mm) coral or aragonite sand substrate is probably the most common and has been around for some time. Finer sand beds over about 1 1/2" start to have the ability to act as nitrate reduction areas. My tanks mainly rely on this type and there are many successful tanks constructed around this type of substrate. This setup also has the advantage that it does not consume excessive amounts of the tank depth like the following two options can. This is probably the easiest, safest and cheapest route to take. Of course reef keepers hardly ever take that route since there is always a better way of doing things right around the corner (or so we hope).
Plenum Systems – were made popular in the 90’s. They have a somewhat complicated construction in which a porus platform is constructed inside the tank that provides about 2" of water dead-space under the sand. The platform is covered with screen to prevent the sand from filling this void but allows water to pass through. The platform is then covered with successive layers of different sizes of sand to form approximately a 4" depth. The logic is that the dead water space provides a nutrient sink and will prevent nitrates from accumulating. This substrate type is starting to lose favor in the reefing hobby for several reasons. One is that it is complicated to setup correctly. Another is that there are many tanks using this system which do not seem to be performing any better than simpler systems and lastly, there have been reports that disturbing the sand bed has caused entire tanks to crash. I will not go into detail on this method since I believe it is a poor choice, but if you are interested in pursuing it, there is a monthly article in FAMA which can get you started.
Deep Sand Bed (DSB)
DSB's are one of the newer things to hit the substrate scene. These consist of one more layers of fairly fine sand that is piled deeply in the bottom of the tank. In some ways, it is similar to a plenum system, but without the plenum and the sand depth is usually deeper. The only known downside to the DSB system is that it occupies a lot of the tank depth and can be somewhat unsightly when viewed from the front. The benefits are reputed to be many. The major one is that it’s depth encourages a wider range of biological processes to occur in the tank than you get with thinner sand beds. This mostly relates to the substrates ability to reduce nitrates to harmless compounds. There are no known long-term problems with DSB substrates yet, unlike plenums, but the technique is still fairly new. To minimize the cosmetic impact, some reef keepers are implementing DSB's in external tanks or sumps. Whether you should use a DSB is mostly dependent on what kind of reef keeper you are. If you want a fairly easy, nice cosmetic setup, I would recommend staying with option ‘b’ above. On the other hand, if you’re a tinkerer and want to be leading edge and you have the tank depth to accommodate it, then DSB is worth looking into. The article 'Muddy Waters' by Dr. Shimek in the Further Reading section below is excellent reading on this topic.
Recently, DSBs have gotten some bad press. Many hobbyists have reported that their tanks decline after the DSBs get several years old and then crash. I do not currently recommend using this method.
BBs are the latest fad. The thinking is that by deleting the sand bed, you are losing some of your filtration, but you make up for it by the fact that there is no place for detritus to collect in the bottom of the tank. Not having a sand bed also means that the amount of water currents in the tank can be increased substantially and so this is gaining some popularity primarily with the SPS hobbyists. The obvious downside is primarily cosmetics. I have tried the BB approach and did not notice a significant difference from running a shallow sand bed.
There is one last spin to the substrate question and that is the topic of 'Live Sand'. Live sand is sand that comes from the ocean presumably with all the varied sand dwelling organisms still in it. The idea is to use some amount of live sand to 'seed' the remaining substrate with sand dwelling organisms. In concept, live sand makes perfect sense to create a thriving sand bed with a myriad of organisms in it in the reef tank very quickly. Many current hobbyists swear by the value of live sand. I am somewhat doubtful as to exactly how much benefit there is to be gained by its addition for two basic reasons. First, any 'dead' substrate seems to quickly be populated by organisms from the live rock. Second, much of the live sand I have seen appears to be pretty sterile looking sand which is stored in conditions that would preclude much of the larger life forms from surviving. If you decide to go this route, make an effort to get as fresh of sand as possible to get the most value for your money.