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Emulating Nature: The Reef Habitat Tank

Model your reef tank after a natural coral reef.

By Article and Photos Richard Harker

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Coral Garden

This beautiful coral garden is a typical patch reef that has corals surrounded with sand.

A common assumption is that the typical reef aquarium is modeled after a natural coral reef — a little slice of reef, if you will. Well, it’s not true. The typical reef aquarium is not actually aquascaped like a real reef. It’s more like bits from many different parts of a reef put together to fit inside a glass or acrylic aquarium. I’d like to see reef aquarists begin to create reef aquariums modeled after real habitats found on real natural coral reefs; but to do that, we have to understand the natural reef.

If we approach a coral reef from the open ocean, we first come upon the reef wall and fore reef. The reef wall is a solid vertical rock structure scoured on top by the full brunt of storms, ocean surge and tidal currents. Not much grows on top of the fore reef. One finds the occasional sturdy coral living within the depressions of an otherwise barren seascape.

Several feet below the surface is where one finds abundant corals, such as sea whips and sea fans. The majority of corals on the reef wall are filter-feeding animals drawing nourishment from the ocean water flowing past the reef wall. The few photosynthetic stony corals found in this environment are modest in size, growing slowly as they cling to the reef wall. The fish life found in this habitat is abundant. This is where one finds numerous surgeonfish, including clown, blue and convict.

Behind the protection of the fore reef lies the reef flat. This is generally where we find the most abundant and diverse aggregations of corals. The calmer waters, combined with plenty of light, create the ideal environment for a wide range of soft and hard corals.

As we move closer to land, the reef flat gives way to patches of rock surrounded by areas of sand. This is the back or patch reef. Sand accumulates in this area because of a combination of environmental and biological processes. As storms pound the most exposed areas of the reef, forces grind coral rock into sand, and currents carry the sand to the protected back reef. This transitional area varies widely, depending on topography, currents and how the area has evolved. Soft corals dominate some back reefs, whereas delicate branching hard corals dominate others. Back reefs in areas under biological stress can be overrun with fleshy marine algae.

Sea Grass

A sea grass aquarium represents a different approach to reefkeeping.

Moving even closer to shore, coral rock gives way to sand, and the sand itself changes, becoming finer and intermixed with detritus. This is a lagoon, the final resting place for all that accumulates on a reef. Lagoons are shallow, by definition. Wind and water currents push sand and detritus from the reefs toward the shore. This builds up over time and gradually becomes a lagoon that is sometimes (not always) attached to a sandy beach. The windward side of an island will be just rock because it gets hit with storms; the leeward side is where one finds lagoons because they are protected from storms (if the area is exposed to strong currents, it isn’t a lagoon).

The sand can often be covered with filamentous algae and cyanobacteria. At first glance, lagoons appear sparsely populated by animals. This is where one finds lone shrimp gobies posted at regular intervals across the sand. Yet a closer look reveals that lagoonal habitats are some of the most biologically diverse habitats of the reef. The great diversity, however, is not found in a lagoon’s waters but rather in the detritus-enriched sand and silt, the sand having high nutrient levels.

There are small transitional areas between each of these habitats, but for the most part, the habitats are visually distinct. One can readily tell the difference between fore reef, reef flat and back reef because of distinct topographies, as well as distinct mixes of animals. Different animals and aggregations of animals have evolved to exploit available niches in each of these habitats.

This is one of the greatest problems of the traditional reef aquarium: Ignoring the linkage between habitats and residents leads a hobbyist to inadvertently risk the well-being of the aquarium’s inhabitants. Placing animals from one habitat into another can potentially stress or impair either the transplanted animals or animals forced to deal with animals they rarely encounter on a natural reef. By modeling a reef aquarium after a single natural habitat, the hobbyist doesn’t just have the satisfaction of creating a reef aquarium that actually looks like a real coral reef — he or she has also created a potentially healthier environment for the saltwater aquarium’s inhabitants.

Building the Habitat Reef Aquarium
From the descriptions of coral reef habitats, I hope it is obvious that the typical “rocks-in-a-glass-box” reef aquarium so common in the hobby is not the place to start when designing a habitat reef aquarium. The traditional reef aquarium unnaturally combines reef wall aquascaping with reef flat inhabitants. A more natural approach is to combine the aquascaping of a specific habitat with the animals that naturally live there.

Regardless of the habitat the aquarist chooses to emulate, this matching is the most important consideration. One can start with the aquascaping, then choose animals that match, or start by choosing the aquarium’s inhabitants, then creating aquascaping that is appropriate for them.

The Natural Lagoon
Despite the beauty and bio-diversity of a healthy lagoon, few hobbyists actually set out to create a lagoon in their reef aquarium. They inadvertently end up with one when they combine poor water conditions, minimal water movement and fine sand. The end result is an accumulation of detritus, algae and cyanobacteria — characteristics of an impaired natural lagoon. Combining lagoonal conditions with fore reef aquascaping and reef flat inhabitants is the problem.

A lagoonal aquarium should be aquascaped with only sand — not live rock. There is a misconception that a reef aquarium needs live rock, but all of the biological benefits of live rock can be provided by live sand. Having only sand and no rock creates a more natural-looking lagoon and has multiple advantages, from initial expense to ease of maintenance.

Good inhabitants for a lagoonal aquarium include shrimp gobies and their commensal shrimp (e.g., Alpheus sp. randalli). These animals constantly build and rebuild burrows in the sand. They need plenty of sand and the kinds of tankmates one would normally find in a lagoon (e.g., Pomacentrus, Neoglyphidodon and Amphiprion). Another option is a lagoon aquarium aquascaped around an anemone and anemonefish. Several anenomes popular in the hobby are only found in sandy lagoonal areas (such as Stichodactyla mertensii, S. haddoni and Heteractis crispa), despite the fact that too many hobbyists insist on filling anemone aquariums with live rock.

The Patch Reef
The back reef, with large areas of sand surrounding the occasional rock structure, is perhaps the most photogenic portion of the reef. The open areas — with plenty of water circulation and intense light — create ideal conditions for stony corals. Unfortunately, most reef aquariums fail to capture this striking portion of the reef. The primary reason is, once again, the typical aquascaping of a reef aquarium. Most aquariums tend to have a low surface area relative to their height. Most reef aquariums are too tall; and because of the height, most hobbyists put too much rock in their aquarium.

Although this approach to aquascaping a reef aquarium is the traditional method, it falls short if one is trying to emulate nature as much as possible. There is no habitat on the natural reef that resembles this stack of rocks. On the natural reef, most saltwater fish spend their time swimming in the open water above the reef. In the typical reef aquarium filled with rock, fish are essentially pressed against the reef structure and have no open areas in which to swim. A reef aquarium filled with rock is also more difficult to maintain. Many of the husbandry problems that develop over time as a reef aquarium ages are compounded by too much rock in the aquarium.

If one’s goal is to re-create a natural patch reef habitat, the first step is to purchase an aquarium with a large surface area. The proportions of the so-called “breeder aquarium” offer the hobbyist a greater opportunity to create a realistic patch reef than the more common “display aquarium.” An even better solution is a low-profile custom aquarium. A large surface area enables the construction of isolated rock clusters surrounded by sand, which characterize the back reef. Large open areas allow fish to swim freely and hover over the rocks, as they would on a natural reef. In addition to being more realistic, the isolated rocks and shallow depth make the aquarium more accessible. From simple chores, such as cleaning the glass, to more labor-intensive tasks, such as vacuuming the sand, every aspect of maintenance is simplified.

A less common type of patch reef is the sea grass patch reef. These areas have an interesting mix of small corals, rubble and sea grass. Environmental requirements for this type of reef aquarium are similar to the patch reef outlined above. Sea grasses tend to be more challenging to grow, so a sea grass aquarium should be reserved for advanced hobbyists; but for those who appreciate a challenge, they are a wonderfully unique display.

The Reef Flat
On a natural reef, as we move back toward the ocean from the patch reef, we reach the reef flat. The reef flat is where we encounter the highest densities of stony corals. So, for a hobbyist primarily interested in creating a reef aquarium that focuses on hard corals, this is the habitat to use as a model.

This habitat presents a number of challenges for aquarists. It is a reef zone that experiences strong currents and intense light. It is also an area regularly flushed by pristine water with low levels of nutrients. And it is an area where rock predominates, and what little sand that exists is coarse gravel too heavy to be carried away by strong currents. In short, it is a habitat that is virtually never re-created in the hobby. The irony is that given the popularity of stony corals, such as Acropora corals, this should be the habitat most frequently emulated.

A reef aquarium designed to mimic the reef flat and create a realistic environment for reef flat stony corals should be built around the unique environmental conditions of this habitat. The aquarium should have intense, turbulent water motion and strong lighting. Like the patch reef aquarium, a aquarium with a substantial surface area is preferable, but the more turbulent water motion requires higher walls and a greater concern for water containment. In fact, all other aspects of the aquarium’s design should be secondary to creating intensely turbulent water motion.

Outlining water motion regimens that mimic reef flat intensities are beyond the scope of this article, but a search of hobby literature will produce a wide range of options. There is the traditional method of employing powerhead pumps, which tend to produce only shallow surface turbulence. Other options, such as the so-called Carlson surge device (named after Bruce Carlson, the professional aquarist who first popularized it), can create much stronger turbulent water flow. Closed loops using powerful pumps can also create strong currents, though these tend to be more laminar than turbulent.

Strong water motion will preclude the use of fine sand. While many in the hobby continue to defend the notion that fine sand helps maintain a healthy reef aquarium, studies show growing evidence suggesting that the opposite is true (e.g., Toonen, R. and C. Wee. 2005. “An Experimental Comparison of Sandbed and Plenum Based Systems” [Parts 1 and 2]. Advanced Aquarists Online). Furthermore, fine sand is only found in lagoonal and near-lagoonal habitats. Fine sands are washed away from reef flats by strong water motion. The inability to keep fine sand on the bottom of the saltwater aquarium is actually a good sign when creating a realistic reef flat habitat. The best substrate for a reef flat is a solid rock base with no sand to trap detritus. Ideally, this would be a slab of rock that virtually covers the bottom of the aquarium and that is placed right on the bottom; one could also simply put down a layer of rock, as if creating a rock sidewalk.

The other requirement for a successful reef flat aquarium is high-quality water. Acropora and other stony corals growing on the reef flat are bathed in clean nutrient-free ocean water. Stony corals produce a great deal of mucus that keeps the coral free of fouling particulate matter. On the natural reef, this nutrient-rich mucus is carried away, ending up as part of the lagoonal detritus. Lacking this export mechanism, the hobbyist is faced with finding some other means to remove these nutrients. Reverse osmosis replacement water, high-capacity foam fractionation (protein skimming) with modest ozone, activated carbon and mechanical filtration are all required to keep the aquarium’s water as pristine as possible.

Your Reef Re-Creation
Re-creating any of the habitats outlined here represents a significant advance over the generic traditional reef aquarium to which we have all become accustomed. Hobbyists contemplating a habitat aquarium are encouraged to closely examine the coral reef photos that accompany this article, as well as read natural history books about coral reefs. With an understanding of the natural reef and a willingness to go beyond the traditional reef aquarium, a hobbyist at any skill level will be able to create a more realistic and ultimately more satisfying reef experience.

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