When setting up a reef tank, aquarists are confronted with a variety of reef rock options. There’s natural, aquacultured or synthetic rock. The same goes for sand substrate. You’ll find everything from super-fine to coarse sand products. Aquarists often wonder if they’re using enough live rock or if the sand bed is too deep or too shallow. Fortunately, it’s not that complicated once you understand the roles played by sand and rock in your aquarium. Some are obvious and some are steeped in myth and mystery. Here’s what you need to know!

Rock & Sand As Decoration

As a reef aquarist you may balk at the idea of “decorations” in your tank. Aren’t “decorations” like sword-wielding skeletons reserved for freshwater aquariums? Maybe a better word is aesthetics. Fine sand gives the bottom of the aquarium a natural appearance. It softens the look of a bare-bottom tank. A light-colored substrate reflects the light and brightens up the aquarium. Rockwork also gives the tank a sense of scale and depth. No matter how you arrange the rocks, it breaks up the “glass box” look.  In addition to all this, natural reefs are made of live rock so we kind of expect a reef tank to have some rock as a base to build on.

Live Rock and Sand as Habitat

It is no secret reef aquariums are home to many kinds of marine life. The most obvious are the large invertebrates like SPS and LPS corals. Live rock provides a platform for arranging coral frags based on size and lighting requirements. Marine fish feel more at ease when they can dart into a rock cave. The rock surface provides a grazing area for fish, shrimp and even live foods like pods.

The sand bed will become colonized by a variety of tiny invertebrates like detritus-eating worms. They may not be exciting to look at but they’re a part of your reef’s ecosystem. Many fish and shrimp will sift through the sand for live foods as well. Some wrasse also need a good layer in order for them to hide and sleep at night, like coris or leopard wrasses.

Biological Filtration

There continues to be debate and discussion about how much sand or live rock is needed to provide enough surface area to support a biological filter. The thinking is by using too little rock or sand your aquarium won’t have enough surface area for the bacteria to colonize. If not, you’ll have chronic levels of ammonia and nitrite. This concept is borrowed from the aquaculture industry. With fish farming one fish species is raised from fry to market size in the shortest amount of time. Aquaculture studies have determined specific formulas for biological filter size based on the fish being raised, protein level in the feed, feeding rate and stocking density in the grow-out system. In contrast, reef aquariums have a very small bioload compared to intensive aquaculture. The truth is, you’ll never run out of surface area for biological filtration in a reef aquarium even with a minimal amount of sand or rock.

What about Denitrification?

Reef aquarists want to keep nitrate levels under control. Proper feeding, protein skimming and mechanical filtration remove organics before they can decompose and contribute to the nitrate load. But there’s also a natural nitrate-removing process that is driven by microbes living in the aquarium. It was once thought that denitrification only occurred under low-oxygen conditions. More recent studies show that nitrate is removed by a variety of microbes at a wide range of oxygen levels. That means natural denitrification occurs on the surface of sand grains, in the pores of live rock, in the filter and refugium with no help from us.

How much sand do I need?

There’s a couple of ways of building a live sand bed. A shallow (1.5 to 2-inch) bed of fine sand looks silky smooth. Sand like Fiji Pink and Aragamax creates this soft look. Keep in mind that small sand grains can be blown around if you’ve got strong flow pumps. This keeps the sand bed clean but may occasionally cause bare spots. If you create a deep sand bed of 6 to 8-inch bed of sand you’ll get the look you want and avoid the bare spots since only the surface layer will be pushed around.   Another method is to create a shallow bed using coarse grained sand. The heavier particles won’t be blown around by circulation pumps.

Sand is typically sold by pre-weighed packages in pounds. The density of the sand depends on the grain size. Our sand bed calculator takes the guesswork out of determining how much you need. Just plug in your tank size, desired depth and density of your favorite sand and the calculator does the rest.

Building a Reef with Rock

Today there are many types of rock to choose from. Some rock are very dense while other materials are relatively light. This makes it almost impossible to come up with a formula based on pounds per gallon. It all comes down to how much structure you want in your tank. Some aquarists like to go with a minimal amount of rock to give their tank an open look. Others construct intricate caves and ridges for fish to explore and to provide different locations to place corals. If you’re setting up a nano reef, start with a small box rubble or a few pieces of branch rock. You can always add more later. The same goes for larger tanks. Try a box or two to see how it looks. Then add accent pieces to create interesting structure for corals and fish. If needed, the rock structure can be held together with glue or epoxy to keep it from toppling over.

Final thoughts

As you can see there is no one size fits all approach. That’s what makes reef-building so much fun. No two reef tanks are ever the same. And remember, it’s OK to take your time and make changes as your tank develops. Use your creativity to create your own unique reef!