Reef aquarium


A reef aquarium or reef tank is a marine aquarium that prominently displays live corals and other marine invertebrates as well as fish that play a role in maintaining the tropical coral reef environment. A reef aquarium requires appropriately intense lighting, turbulent water movement, and more stable water chemistry than fish-only marine aquaria, and careful consideration is given to which reef animals are appropriate and compatible with each other.

Components

Reef aquariums consist of a number of components, in addition to the livestock, including:
  • Display tank: The primary tank in which the livestock are kept and shown.
  • Stand: A stand allows for placement of the display tank at eye level and provides space for storage of the accessory components.
  • Sump: An accessory tank in which mechanical equipment is kept. A remote sump allows for a clutter-free display tank.
  • Refugium: An accessory tank dedicated to the cultivation of beneficial macroalgae and microflora/fauna. The refugium and sump are often housed in a single tank with a system of dividers to separate the compartments.
  • Lighting: Several lighting options are available for the reef-keeper and are tailored to the types of coral kept.
  • Canopy: The canopy houses the light fixtures and provides access to the tank for feeding and maintenance.
  • Filtration and water movement: A variety of filtration and water movement strategies are employed in reef aquaria. Bulky equipment is often relegated to the sump.

    Display tank

A "reef ready" or simply "drilled" tank is often used. This style of tank has holes drilled into the rear pane allowing water to drain into the sump or refugium.
These drains are usually housed in an internal overflow apparatus made of plastic or glass which encloses a drain standpipe and a water return line. The surface water pours over the overflow, down the standpipe, through PVC piping, into the sump. After transiting the sump, water is pushed by a return water pump through the second hole and into the aquarium.
Alternatively, standard non-drilled aquariums employ an external "hang-on" overflow that feeds water via continuous siphon to the sump to fig 1.
The tanks are usually constructed from either glass or acrylic. Acrylic has the advantage of optical clarity, lightness, and ease of drilling. Drawbacks include a tendency to scratch easily, bowing, and often limited access from above due to top bracing. Glass aquariums are heavier but harder to scratch. Other materials such as epoxy-coated plywood have been used by industrious DIYers, but these materials are typically reserved for the construction of larger tanks.

Filtration

The primary biological filtration for reef aquariums usually comes from the use of live rock which come from various tropical zones around existing reefs, or more recently aquacultured rock from Florida. Some reefkeepers also use what is called deep sand beds. These are often employed to augment the biological filtration by aiding in the reduction of nitrate, a waste product in an incomplete nitrogen cycle. Deep sand bed opponents may prefer a "bare bottom" or "suspended reef" which allows for easier removal of nitrate-generating accumulated detritus. This biological filtration is usually supplemented by protein skimmers. Protein skimmers use the foam fractionation process wherein air is introduced into a water stream creating microbubbles. Organic waste adheres to the surface of these microbubbles and is removed as it overflows at the reactor surface into a removable cup. This group of elements used in conjunction is characteristic of the Berlin Method, named for the city in which it was first devised.
In recent years, the Berlin Method is often supplemented with a refugium. A refugium provides many benefits, which include nitrate reduction, as well as providing a natural food source. It typically houses two main species of macroalgae, including Caulerpa prolifera or chaetomorphae or both. Macroalgae is used for two reasons: to remove excess nutrients from the water such as nitrate, phosphate, and iron, and to support beneficial microflora and fauna. Small invertebrates are provided a space free of predation to grow and, when returned to the display tank, serve as food for corals and fish. Conventional combined mechanical/biological filtration used in fish only systems is avoided because those filters trap detritus and produce nitrates which may stunt or even kill many delicate corals. Chemical filtration in the form of activated carbon is used when needed to remove discoloration of the water, or to remove dissolved matter to help purify the water in the reef system.

Water movement

Water movement is important in the reef aquarium with different types of coral requiring different flow rates. At present, many hobbyists advocate a water turnover rate of 10x: 10 x aquarium capacity in gallons = required flow in gallons per hour⁠—⁠this is mathematically equivalent to a complete aquarium water turnover every 6 minutes. This is a general rule with many exceptions. Some corals, such as mushroom corals and polyp corals, require very little flow to thrive. Conversely, large-polyp stony corals such as brain coral, bubble coral, elegance coral, cup coral, torch coral, and trumpet coral require moderate amounts of flow, and small polyp stony corals such as Acropora, Montipora, Porites, and Pocillopora require high, turbulent, conditions, which imitates breaking waves in shallow water near the tip of the reef. The directions which water pumps are pointed within an aquarium will have a large effect on flow speeds. Many corals will gradually move themselves to a different area of the tank if the water movement in its current area is not satisfactory.

"Since flow speed is the critical measure for determining the rate of gas exchange, turnover does little to convey how fast a coral will respire and photosynthesize."

Reef ready tanks obtain at least a portion of the required water motion from the pump that returns water from the sump. This flow usually is augmented by other strategies. A popular strategy is placement within the display tank of multiple powerheads. Powerheads are simply small submersible water pumps that produce a laminar or narrow, unidirectional water stream. If the presence of the powerhead in the tank does not fit with the aesthetics of the display, small holes may be drilled in an overflow of a tank and the bulk of the powerhead can be hidden, leaving only the small funnel spout visible in the tank. The pumps may be alternately switched on and off using a wave timer and aimed at one another or at the aquarium glass to create turbulent flow in the tank. Drawbacks to the use of these powerheads include their capacity to clutter the display tank, propensity for excess heat production, and the laminar quality of water flow often produced. Another method is the closed loop in which water is pulled from the main tank into a pump which returns the water back into the aquarium via one or more returns to create water turbulence. Newer submersible propeller pumps are gaining popularity and are able to generate large volumes of turbulent water flow without the intensely directed laminar force of a power head. Propeller pumps are more energy-efficient than powerheads, but require a higher initial investment.
Another recent method is the gyre tank. A gyre tank encourages a maximum amount of water momentum through a divider in the center of the aquarium. The divider leaves an open, unobstructed space which provides a region with little friction against water movement. Building water momentum using a gyre is an efficient method to increase flow, thus benefiting coral respiration and photosynthesis.
Water flow is important to bring food to corals, since no coral fully relies on photosynthesis for food. Gas exchange occurs as water flows over a coral, bringing oxygen and removing gases and shedding material. Water flow assists in reducing the risk of thermal shock and damage by reducing the coral's surface temperature. The surface temperature of a coral living near the water's surface can be significantly higher than the surrounding water due to infrared radiation.

Lighting

With the advent of newer and better technologies, increasing intensities and a growing spectrum, there are many options to consider.
Many, if not most aquarium corals contain within their tissue the symbiotic algae called zooxanthellae. It is these zooxanthellae that require light to perform photosynthesis and in turn produce simple sugars that the corals utilize for food. The challenge for the hobbyist is to provide enough light to allow photosynthesis to maintain a thriving population of zooxanthellae in a coral tissue. Though this may seem simple enough but in reality can prove to be a very complex task.
Some corals, such as mushroom corals and polyp corals, require very little light to thrive. Conversely, large-polyp stony corals such as brain coral, bubble coral, elegance coral, cup coral, torch coral, and trumpet coral require moderate amounts of light, and small polyp stony corals such as Acropora, Montipora, Porites, and Pocillopora require high intensity lighting.
Of the various types, most popular aquarium lighting comes from metal halide lamps, very high output or VHO, compact fluorescent and T5 high output lighting systems. Although they were once widely used, many reef tank aquarists have abandoned T12 and T8 fluorescent lamps due to their poor intensity, and mercury vapor due to its production of a limited light spectrum.
Recent advances in lighting technology have also made available a completely new technology for aquarium lighting: light emitting diodes. Although LEDs themselves are not new, the technology has only recently been adapted to produce systems with qualities that allow them to be considered viable alternatives to gas- and filament-based aquarium lighting systems. The newness of the technology does cause them to be relatively expensive, but these systems bring several advantages over traditional lighting. Although their initial cost is much higher, they tend to be economical in the long run because they consume less power and have far longer lifespans than other systems. Also, because LED systems are made of hundreds of very small bulbs, a microcomputer can control their output to simulate daybreak and sunset. Some systems also have the ability to simulate moonlight and the phases of the moon, as well as vary the color temperature of the light produced. Moreover, some manufacturers produce LED lighting systems in single bright and double bright intensities for sustaining coral life in marine aquariums.
The choices for aquarium lighting are made complicated by variables such as color temperature, color rendering index, photosynthetically active radiation and lumens. Power output available to the hobbyist can range from a meager 9 W fluorescent lamp to a blinding 1000 W metal halide lamp. Lighting systems also vary in the light output produced by each bulb type—listed in order of weakest to strongest, they would be: T8/12 or normal output lamps, compact fluorescent and T5 high output, VHO, and metal halide lamps. To further complicate matters, there are several types of ballasts available: electric ballast, magnetic ballast, and pulse start ballast.