Reef tank

Reef aquarium

A reef aquarium or reef tank is an aquarium containing live corals and other animals associated with coral reefs.

These aquarium setups vary from having many fish that vary from chlorinating fish and spawning fish, which attempt to recreate life specific to one region of the world like the Great Barrier Reef, to the more prevalent and often spectacularly colored mixed reef that blend hard and soft corals from around the world.

Unlike the marine aquarium, which are built to house various types of fish, the main attraction in many reef tanks are the varieties coral and other invertebrates.


Reef aquariums consist of a number of components including:


Regular glass or acrylic style aquariums are used for reef aquariums; these usually include an internal overflow made of plastic or glass which encloses holes that have been drilled into the bottom glass to accommodate a drain or standpipe and a return line (See fig 1, a). Water pours over the overflow into and down the standpipe(See fig 1, b), through PVC piping, into a sump, which generally houses various filtration and heating equipment, through a return water pump and finally back via more piping through the second hole into the aquarium (See fig 1, c). Alternatively, aquariums sometimes employ an external "hang-on" overflow with a U-tube that feeds water via continuous siphon to the sump(See fig 1 d) which returns it via a water pump.


The primary filtration for reef aquariums usually comes from the use of large amounts of live rock which come from various rubble zones around existing reefs or more recently aquacultured rock from Florida which is supplemented by protein skimmers. This method first came from Germany and is termed the Berlin Method. In addition, a refugium which houses many species of macroalgae, including Caulerpa racemosa or chaetomorphae macroalgae is sometimes used to remove from the water excess nutrients such as nitrate, phosphate, and iron. Some aquarists also advocate the use of deep sand beds.

More usual combined mechanical/biological filtration is avoided because these filters trap detritus and produce nitrate which may stunt the growth or even kill many delicate corals. Chemical filtration is used sparingly to avoid discoloration of the water, to remove dissolved matter (organic or otherwise) and to help stabilize 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 a general rule with many exceptions. For instance, Mushroom Coral requires little flow and is commonly found in crevices near the base of the reef. Species such as Acropora and Montipora thrive under much more turbulent conditions in the range of 30 to 40 times more flow, 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.
"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.

To create turnover many reef aquarists use an Overflow (internal or external) which drains water into a sump where it is then pumped back into the tank. Tanks that come equipped with an internal overflow and pre-drilled holes are known in the hobby as "Reef Ready" or simply "Drilled" tanks. Of the many methods of creating the required flow, one of the most popular is by using multiple powerheads which are simply small submersible water pumps. The pumps may be randomly switched on and off using a wave timer, with each aimed at the flow of another powerhead or at the aquarium glass to create flow in the tank. Another method gaining popularity 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. Only recently available commercially, submersible propeller pumps are gaining popularity due to being able to generate large volume of water flow (turbulent flow) without the intense directed force (laminar flow) 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 gasses 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.


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, in reality this can prove to be a very complex task.

Some corals such as the Mushroom Coral and Coral Polyps require very little light to thrive – conversely, LPS coral such as Brain coral, Bubble Coral, Elegance Coral, Cup Coral, Torch Coral, and Trumpet Coral reqiure moderate amounts of light, and Small Polyp Stony Corals (SPS) such as Acropora Coral, Montipora, Porites, Stylopora and pocillopora require high intensity lighting.

Of the various types, most popular aquarium lighting comes from metal halide, 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: lightemitting diodes (LEDs). 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 comprised of hundreds of very small bulbs, their output can be controlled by a microcomputer 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.

The choices for aquarium lighting are made complicated by variables such as color temperature, (measured in kelvins), color rendering index (CRI), photosynthetically active radiation (PAR) and lumens. Power output available to the hobbyist can range from a meager 9 W fluorescent lamp to a blinding 1000 W metal halide. 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.

Luckily, the choice of lighting systems for a hobbyist can usually be narrowed by first determining which types of corals the hobbyist plans on keeping, since this is the primary factor in determining lighting needs.

Heating & cooling

Most hobbyists agree that a reef tank should be kept at a temperature between 25 and 27 °C (75-80 ºF). Radical temperature shifts should be avoided as these can be particularly harmful to reef invertebrates and fish. Depending on the location of the tank and the conditions therein (i.e. heat/air conditioning), you may need to install a heater and/or a chiller for the tank. Heaters are relatively inexpensive and readily available at any local fish store. Aquarists frequently use the sump to hide unsightly equipment such as heaters. Chillers, on the other hand are expensive and are more difficult to locate. For many aquarists, installing surface fans and running home air conditioning suffice in the place of a chiller. Fans cool the tank via evaporative cooling and will require more frequent top off of the aquarium.

Nano reefs

A nano reef is a type of Marine aquarium that is typically less than 30 gallons. The exact limit that distinguishes a nano reef from a regular reef is somewhat ill-defined (some claim that anything less than 40 gallons would qualify), but 30 gallons seems to be the generally accepted limit. Nano reefs have become quite popular in recent years among fish keeping hobbyists, primarily because of their smaller size, maintainability, and the possibility of lower costs. The burgeoning interest in this niche of marine aquarium science has fostered several notable contributions ranging from specific consumer products such as specialized aquarium filters, compact high intensity lighting systems and smaller circulation pumps. Such equipment allows the aquarist to maintain an environment wherein many marine organisms are capable of thriving.

Pico reefs

Another term gaining popularity is pico reef, which is used to refer to the smallest of nano reef aquariums. These tiny tanks require even more diligence with regard to water changes and attention to water chemistry because the small water volume provides little room for error. Care must be exercised when stocking these tiny tanks because too many inhabitants can easily overload the tank's ability to process wastes effectively. For the smallest of pico reefs, even the presence of a single fish is discouraged. Pico reefs often consist of live rock, hardy corals, and small invertebrates such as hermit crabs and marine snails.

Challenges associated with small reef aquariums

Because of the small water volume, nano reef aquariums require extra attention to water quality compared to aquariums of larger water volumes. Many experienced reef aquarists recommend testing the water twice weekly, with water changes at least every week. In particular, ammonia, nitrite, nitrate, pH, salinity, alkalinity, calcium and phosphate levels should be monitored closely. When it comes to nano reefs, even minute changes in water conditions such as mild temperature fluctuations can be problematic, whereas the greater water volume of larger aquariums provides a more stable and flexible environment.

Nano reefs also require extra care in the selection of occupants. There are two major factors to be considered: biological load, i.e. the ability of the tank to process the wastes produced by the occupants, and species compatibility. These issues, though present in larger tanks, are magnified in the nano tank. Species considered reef safe and able to coexist in larger tanks may not do well in a nano tank due to their close physical proximity. For this reason, smaller species of fish such as gobies and clownfish are popular choices due to their relatively small size and ability to coexist peacefully with other tank inhabitants.


Many nano reef aquarists prefer their nano reefs to be as natural-looking as possible, and therefore choose to use as few mechanical filtration methods as possible. The primary filtration method of choice in nano reefs is a good amount live rock and live sand, which are pieces of rock that have broken from the coral reef and are populated with millions of beneficial bacteria and organisms which aid in the dissolving of organic wastes produced by larger organisms in the nano reef.

However, it is largely recommended that nano reef aquarists use devices such as protein skimmers to remove excess wastes from the aquarium Before it has a chance to be broken down in to nitrate.

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