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Marine aquarium

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A marine aquarium is an aquarium that keeps marine plants and animals in a contained environment. Marine aquaria are further subdivided into fish only (FO), fish only with live rock (FOWLR), or reef aquariums. Marine fishkeeping is different from its freshwater counterpart because of its complexity; the marine environment is more difficult to maintain, requiring more equipment and time from the hobbyist, and the aquarium inhabitants are often more expensive to acquire. Saltwater aquarium enthusiasts are also major contributors to the vast scientific knowledge shared within the hobby.
Marine Aquarium - photograph by John Catsoulis
Marine reef aquarium at the London aquarium
Marine reef aquarium at the London aquarium


Marine fishkeeping history

The very first saltwater tanks were glass jars where the Romans kept anemones outside but were very short lived. The first personal saltwater fishkeeping began on a wider scale in the 1950’s, starting with the basic rectangular glass aquariums (usually 20 gallon), still popular today. Bleached coral along with a substrate of coarse crushed coral was the norm. Algae, including beneficial such as coralline, were viewed negatively and generally removed. The clean, sterile tank was viewed as the healthiest.

During the beginning days of marine aquaria, saltwater was initially collected at local beaches. Natural saltwater contains many unwanted organisms, along with the occasional unwanted pollutant. Aquarium literature of the time suggests that the most commonly kept marine fish of the day were the Percula Clownfish, Sergeant Major Damselfish, small, brackish pufferfish and scat, Jeweled Blennies, Sailfin Mollies, and Blue Damsels. Aquariums were equipped with large air compressors, and were heavily aerated and filtered (primarily with undergravel filters, a norm for some time).

Later in the hobby, air driven, counter-current protein skimmers were invented and revolutionized in Germany along with the Eheim pump company. Perhaps the largest revolution in fishkeeping was a more reliable submersible electric heater, invented by Eugen Jäger. Even today, Jäger is still a major company in aquarium heating.

Various initial aquarists attempted to find the chemical properties of sea water and mix in necessary trace elements to create synthetic salt mixes. Perhaps the first and undoubtedly the largest synthetic sea salt company was Instant Ocean. This revolutionized marine fishkeeping in landlocked areas instead of restricting it to areas near sources of seawater.

Various advancements in filtration included the trickle and hang-on filters, both allowing a more natural equilibrium to the aquarium environment. The advancement of fluorescent lighting technologies into higher outputs along with metal halide lighting established the reef tank, making it a possible to keep corals and invertebrates without natural sunlight.

More efficient chemical testing and more advanced knowledge allowed aquarists to have an idea about the chemical conditions and properties of aquariums. The biological establishment and understanding of maintaining an artificial ocean environment brought more successful and widespread marine fishkeeping. In the 1980’s, the multitude of aquarium publications had greatly increased, and general chemical and biological knowledge was more widespread.

Modern fishkeeping

Marine Aquarium Components

The major components of a marine aquarium are a tank, usually composed of glass or acrylic, a stand, components necessary for chemical, mechanical, and biological filtration, lighting, and an apparatus used to heat the aquarium, usually an aquarium heater. Marine Aquariums can range anywhere from a tiny, nano reef aquariums to gigantic aquariums in surplus of 300 U.S. gallons (1200 L). Nano tanks are usually between 2.5 and 30 US gallons (10 and 120 L). These tanks are difficult to maintain due to the highly variable chemical properties of saltwater. It is usually easier to make mistakes with a highly concentrated body of water. However, large tanks, on the contrary, while easier to maintain on the biological and chemical level, are usually a financial impossibility for most individuals. For this reason, most saltwater tanks are between 40 and 100 US gallons (160 and 400 L). The stand is usually made of wood: fish tanks filled with water are extremely heavy and need a very sturdy base (a 75 US gallon aquarium filled with water and substrate can weigh in surplus of 600 US pounds).

The new age of fishkeeping focuses on various components functioning in order to create a biologically stable environment. There are various orthodoxies of fishkeeping today. The basic varieties of tanks are fish only (FO), fish only with live rock (FOWLR—arguably the most popular) and the reef tank, containing corals, sea anemones, or other delicate invertebrates, using high powered fluorescent or metal halide lighting fixtures in order to acquire the needed light output.

Live rock

Live rock is rock that has been in the ocean, composed of limestone and decomposing coral skeleton, usually around a coral reef such as those around Fiji, and is usually covered with beneficial algae, coralline and tiny invertebrates and bacteria that are desirable in the aquarium. Some examples of the microfauna commonly found on live rock are crab, snail, feather dusters, brittle stars, sea stars, limpets, abalones, and an occasional sea urchins, anemones, coral, and sea sponge. Bristleworms are also common, most of which, while unattractive, are not harmful and are useful scavengers; some species can be pests, however. The addition of live rock is one of the best ways to ensure a healthy aquarium, as the rock provides a buffer to maintain high pH (8.0-8.3), alkalinity, and acid-neutralizing capacity. Alkalinity is often known by a rather confusing term, "carbonate hardness", or KH. This is usually measured in "degrees" (dKH) or meq/L.

The microfauna found on live rock are detrivores and herbivores (as they eat algae and fish waste), and provide fish with a natural, attractive shelter. Live rock usually arrives from online dealers as "uncured", and must be quarantined in a separate tank while undergoing the curing process, which involves the inevitable die-off of some of the rock's inhabitants and the subsequent production of undesirable ammonia and nitrite. Live rock that is already cured is available at most pet stores that cater to saltwater. Live sand is similar to live rock and is equally desirable.


There are a multitude of filtration methods available to the modern aquarist. All methods use a combination of chemical, mechanical, and biological filtration. Common hang-on filters usually only offer chemical and mechanical filtration through mesh and carbon, which is not enough for the saltwater aquarium, although some hang-on filters have plates where bacteria can colonize and provide biological filtration as well. These plates should never be cleaned, as this would kill the bacteria colonies. Most filters are marked to provide which forms of filtration they accommodate.

Depending on what is in the tank, protein skimmers are also very useful in the reef tank. With a low bioload (not many animals in the tank) and hardy livestock, a protein skimmer may not be absolutely necessary, though it is never a detriment. A heavily stocked tank, corals, and other delicate animals may need a protein skimmer to survive.

Perhaps the most popular and widespread biological filter is the refugium. Water is drawn from the main tank to the refugium (usually a smaller container hidden behind or below the main aquarium), then is returned to the main tank. This smaller tank serves two purposes: it adds water volume (thus diluting any chemical problems), and it provides a place for biological filtration. Refugiums can also serve as a temporary shelter area for recovering injured fish, delicate animals that need to be slowly acclimated to the aquarium environment, and to temporarily separate fighting inhabitants. Refugiums often contain live rock and live sand, macroalgae, and scavenger microfauna such as micro brittle stars, tiny sea stars such as asterina, snails, and worms. Some people also use refugiums to raise tiny brine or mysis shrimp for delicate fish like seahorses and dragonets. A sump is a refugium that also contains the main aquarium's other equipment, to keep all hoses, filters, and heaters out of view. This is especially common for show tanks and reef tanks.


Lighting is a controversial topic in fishkeeping. Most fish are not particular about lighting—the light is more for the aquarist than for the fish. However, a regular cycle of light and dark, simulating day and night, is beneficial for fish, since it establishes a routine and makes them feel more secure. Any light will do for a fish only (FO) tank. A fish only with living rock (FOWLR) tank will need a more powerful light to encourage the growth of coralline algae. There are many types of lights available; generally speaking, the higher the power rating in watts, the better. Some types include fluorescent, VHO fluorescent (Very High Output), compact fluorescent and metal halide. A powerful fluorescent lighting fixture may be enough for a fish only with living rock (FOWLR) system, depending on the depth and size of the aquarium. Coral reefs require more powerful lights, the general rule being 3 to 5 watts of power per gallon of water; research of particular species' needs before buying is essential. Metal halides are the most popular lighting fixture of reef aquariums, since they provide more light per lamp than fluorescent fixtures. Additionally, metal halide lamps produce light from a single "point source" and thus produce flickering "glimmer lines" on the aquarium floor, an effect prized by aquarists. Actinic lights produce a deep blue spectrum designed to simulate the dominant wavelength of light a few metres below the ocean's surface.

Recent advances in lighting technology have also made available a completely new technology in aquarium lighting; Light Emitting Diodes (LEDs). Although LEDs themselves are not new, the lighting temperature which used to very low on LEDs has now been able to increase to a point that LEDs can now be used as a viable alternative to gas and filament based lighting. LED lighting is still in its early stages and therefore very expensive, however there are several advantages that these lights bring over traditional lighting, barring the initial cost; they consume very little power, in being solid-state they have an exponentially longer lifespan- and do not need to be replaced, and because the "light fixture" is comprised of hundreds of small bulbs, these can installed and controlled by a microcomputer to slowly fade in and out to simulate day break and sunset, as well as having the ability to mix in special blue LEDs which can simulate the moon at night, also with the programmability of having the phases of the moon.


Most fishkeeping is tropical, and most fish gathered are caught in Africa, Southeast Asia, and the Red Sea. A temperature of 75 degrees Fahrenheit to 82 degrees Fahrenheit (24 °C to 28 °C) is the most common, reflecting the environment of the organisms. This generally necessitates the use of a heater, almost always fully submersible, though reef tanks with very powerful (and therefore hot) lights can actually require cooling in some locations. Cold water marine fishkeeping is generally only regional, and most cold water fish are caught locally. This is because the physical appearance of cold water fish is not as attractive or colorful as that of their warmer-water counterparts.

Water testing

The most common things to test for in a saltwater aquarium are:

Specific gravity should be kept between 1.020 and 1.024 for a fish only tank, and 1.023 and 1.026 for invertebrates and corals. Salinity should therefore be between 28-32 PPT. Salinity is directly related to specific gravity and both can be tested with a hydrometer, which is inexpensive but often imprecise. A more precise way of measuring the salinity is to use a refractometer which, once properly calibrated, offers greater precision.

pH should be maintained between 8.1 and 8.3 (can be raised with a commercially available buffer or through calcium-rich substrata); Carbonate hardness (KH) should be between 8 and 12 degrees. The best way to avoid KH and pH swings is to purify the water with a reverse osmosis / deionization (RO/DI) unit.

The nitrogen cycle refers to the conversion of toxic ammonia, to nitrite and finally nitrate. While fish waste (urine and feces), and decaying matter release ammonia, the majority of ammonia (approximately 60%) in both marine and freshwater aquariums is excreted directly into the water from a fishes' gills. Biological (bacterial) nitrification converts the ammonia into nitrite ions, NO2-, and then to nitrate ions, NO3-. Nitrate is readily taken up and assimilated by algae and hermatypic corals. Some nitrate can be converted by an anaerobic bacterial process to free nitrogen, but this process is very difficult to maintain. Most nitrate, which is less toxic to fishes and most invertebrates accumulates in the water until it is physically removed by a water change. Ammonia and nitrite should be tested regularly; any detectable levels (i.e., over 0 ppm) are indicative of a problem. Nitrate should not exceed 20ppm in reef tanks, or 40ppm in fish-only tanks. It is normal to have a little nitrate buildup, and some livestock handle it better than others. Most hermatypic corals, while able to assimilate nitrate, cannot be expected to perform well with chronically high nitrate concentrations (>40 mg/L as nitrate ion (~ 10 mg/L nitrate-nitrogen)).

Other suggested tests include calcium, alkalinity, iodine, strontium, molybdenum, and other trace minerals. Research the particular species you wish to keep to see if it is necessary for you to do additional testing.

Water changes

Water changes are a staple of good saltwater maintenance. It involves removing from 10% to 20% of the total volume of the aquarium, replacing that water with new pre-mixed saltwater. Pre-mixed saltwater has been dechlorinated and/or dechloraminated, brought to the temperature and salinity of the aquarium, and left in a mixing container (such as a bucket with a powerhead) for long enough to ensure that all the salts have been dissolved (typically several hours).

Replacement water should be of the same source as the aquarium, whether it be reverse osmosis, de-ionized, distilled or municipal supply, in order to avoid drastic changes in water chemistry. In cases where one is replacing a tap water-based salt mix with a reverse osmosis-based salt mix, the replacement water should be added slowly over the course of several hours to avoid sending the aquarium inhabitants into osmotic shock. If using municipal water, one should check with the local utility company to find out the composition of that tap water. Water containing high levels of nitrate or phosphate should be avoided, and reverse osmosis or distilled water used in its place.


Almost all species kept in marine aquaria at this time are caught in the wild. Very few species, such as clownfish, are captive-bred on a commercial scale. Much collecting is done in Indonesia and the Philippines, where use of cyanide and other destructive collection methods is common. The majority of live rock is also harvested in the wild, and recent restrictions on this harvest in Florida have caused a shift to Fijian and aquacultured rock. Natural rock takes many years if not centuries to form, and is vital habitat for countless marine species, and thus, commercial-scale harvesting of naturally-occurring live rock has been criticized by conservationists. Additionally, many animals species sold to hobbyists have very specific dietary and habitat requirements that cannot be met by hobbyists (e.g. Labroides genus wrasses, the Moorish Idol); these animals almost inevitably die well before their time, and their color and appearance is poor. These issues are often downplayed by individuals and organizations with a financial interest in the trade. Hobbyists should be urged to buy only certified net-caught fish (although ensuring the legitimacy of such claims can be difficult) or captive-raised fish, as well as farmed corals and to support legitimate reef conservation efforts. It should be noted that the majority of corals can be "fragged", whereby a portion of a larger captive coral is separated and can subsequently be raised into an individual specimen, allowing for coral propagation withing the domestic aquarium; the trade in frags offers a fantastic opportunity for marine aquarists to obtain new and unique corals whilst limiting the effect on the natural environment. Rare species and those without a history of being successfully kept in captivity should be avoided.

Commercial front

Various businesses have brought a commercial front to fishkeeping, perhaps the largest being Marineland, Inc. Along with movies such as Finding Nemo, fishkeeping is becoming much more widespread than ever before. Perhaps the biggest turndown in fishkeeping is the initial cost. A 100 US gallon (400 L) reef tank full of coral and equipment can cost in excess of $5000 US. Aside from the difficulty, this is a large factor as to why freshwater fishkeeping is still so widespread in comparison to its marine counterpart.

References and further reading

  • The Conscientious Marine Aquarist, by Robert Fenner
  • Invertebrates: A Quick Reference Guide, by Julian Sprung
  • Corals: A Quick Reference Guide, by Julian Sprung
  • Reef Invertebrates: An Essential Guide to Selection, Care and Compatibility, by Anthony Calfo and Robert Fenner
  • Aquarium Corals : Selection, Husbandry, and Natural History, by Eric H. Borneman
  • Natural Reef Aquariums: Simplified Approaches to Creating Living Saltwater Microcosms, by John H. Tullock

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