A marine aquarium is an aquarium that keeps marine plants and animals in a contained environment. Marine aquaria are further subdivided by hobbyists into fish only (FO), fish only with live rock (FOWLR), and reef aquaria. Marine fishkeeping is different from its freshwater counterpart because of the fundamental differences in the constitution of saltwater and the resulting differences in the adaptation of its inhabitants. A stable marine aquarium requires more equipment than freshwater systems, and generally requires more stringent water quality monitoring. The inhabitants of a marine aquarium are often difficult to acquire and are usually more expensive than freshwater aquarium inhabitants.

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 1950s, 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 scats, jeweled blennies, 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 1980s, 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 are an aquarium, usually made from glass or acrylic, filtration equipment, lighting, and an aquarium heater. Marine aquariums can range in volume from less than 80 litres, (

Types of marine aquarium
Marine aquarists typically divide saltwater aquariums into those housing fish only, fish with live rock and those primarily designed to house corals and other invertebrates (also known as Reef aquariums).

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, starfish, limpets, abalones, and an occasional sea urchins, anemones, coral, and sea sponge. Also, if the aquarist is unlucky in most cases, a mantis shrimp. 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.

Filtration
Main article: Filter (aquarium)
In general, marine aquariums have more complex filtration requirements than most freshwater aquariums. The various components frequently include Wet and dry filters and Protein skimmers. Protein skimmers are devices that remove organic compounds prior to their degradation, and are also very useful in marine aquariums. Protein skimming is also used in the popular Berlin method that relies on live rock, and periodic partial water changes to degrade and remove waste products. The Berlin method relies on large amounts of live rock being included in the aquarium. The rule of thumb is 1/2 – 1 lb. per 1 US gallon (0.2 – 0.4 kg per 4 liters). Some marine aquariums also include a refugium. Refugiums are small containers, or aquariums hidden behind or beneath the main aquarium and connected to it via a water pump. Refugiums have recently become quite popular among reef aquarists. Refugiums serve several purposes: adding water volume, providing a fish-free site for biological filtration in live rock and/or the sandbed. Fish-free refugiums are host to populations of copepods, amphipods, isopods and other zooplankton.

LIGHTING

Regular cyclical lighting is used in aquariums to simulate day and night. This is beneficial for fish and invertebrates since it establishes a routine, enables them to rest, and makes them feel more secure. The lighting used varies depending on the inhabitants of the aquarium. Typically, the type of lighting for aquariums with fish only is regarded as unimportant. In aquariums containing invertebrates, however, where algal growth (of both free-living and symbiotic algae) is desired, more intense lighting is required. There are many types of lights available: some common types include fluorescent, VHO fluorescent (Very High Output), compact fluorescent, LED and metal halide. Actinic lights produce a deep blue spectrum designed to simulate the dominant wavelength of light a few metres below the ocean’s surface.
When considering lighting for an aquarium, there are generally two factors to consider: wattage and color temperature. Depending on the type of lighting (i.e. flourescents, metal halide, etc) the wattage of light emitted may vary considerably, from tens of watts to several hundred watts in a lighting system. Wattage, while not indicative of color, is equivalent to power and essentially determines how brightly the light will shine. Due to the scattering of light in water, the deeper one’s tank is, the more powerful the lighting required. Color temperature, measured in kelvin (albeit slightly unrepresentively) refers to the color of light being emitted by the lamp and is based on the concept of blackbody radiation. Light from the sun has a color temperature of approximately 5900 K and lighting systems with color temperatures >5000 K tend to be best for growing plants in both the marine and freshwater setting. 10,000 K light appears bluish-white and emphasizes coloration in fishes and corals. Higher up on the spectrum there are 14,000 K and 20,000 K bulbs that produce a deep blue tint which mimic the lighting conditions underseas, creating an optimal ambience for invertebrates and livestock present.

Heating
Most marine aquarium inhabitants are endemic to tropical reefs and waters in Africa, Southeast Asia, and the Red Sea. Marine aquarium temperatures should mimic the natural environment of the inhabitants and are most commonly maintained at 24 to 28 degrees Celsius (75-82 °F). In regions where the ambient temperature is less than the desired temperature of the aquarium this generally necessitates the use of an aquarium heater. In some areas ambient temperature is greater than the desired temperature and refrigeration devices, known as “chillers”, are used to cool the aquarium water.

Water testing
Marine aquarists commonly test the water in the aquarium for a variety of chemical indicators of water quality. These include:
Specific gravity, a relative measure of water density, is normally maintained between 1.020 and 1.024 in aquariums with fish only, and 1.023 and 1.026 for aquariums containing invertebrates. Salinity should therefore be between 28 and 32 PPT. Because salinity is by definition directly related to specific gravity, both can be tested with an inexpensive hydrometer or refractometer.
pH should be maintained between 8.1 and 8.3. This can be raised with a commercially available buffering agent or through calcium-rich substrata. Carbonate hardness (KH) should be between 8 and 12 degrees. A calibrated calcium reactor can assist in maintaining both pH and carbonate hardness. Using purified water from a reverse osmosis / deionization (RO/DI) unit can prevent KH and pH fluctuation.
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 released (approximately 60%) in both marine and freshwater aquariums is excreted directly into the water from the 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 is converted via 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 than nitrites, 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) can be indicative of a problem. Nitrates should not exceed 20ppm in reef tanks, or 40 ppm in fish-only tanks. It is normal to have a small amount of nitrate buildup, and some livestock are more capable of living in these conditions than others. Most hermatypic corals, while able to assimilate nitrate, cannot be expected to survive indefinitely with chronically high nitrate concentrations (>40 mg/L as nitrate ion (~ 10 mg/L nitrate-nitrogen)).
Other suggested tests include those for calcium, alkalinity, iodine, strontium, molybdenum, and other trace minerals. It is often beneficial (and necessary) for the aquarist to research the water chemistry parameters for the specific organism that is desired.

Water changes
Water changes are a staple of good saltwater maintenance. Larger (approximately 200 gallon) aquariums are much more stable and water changes may not need to take place if the nitrogen cycle has fully established itself in the tank, although this a controversial statement among aquarists. Supplements are sometimes needed (such as calcium). Water changes involve removing a fraction of the total volume of the aquarium, replacing that water with new pre-mixed saltwater. Pre-mixed saltwater has been dechlorinated and/or dechloraminated–typically with an additive such as bisulfite or through filtering. Water should be brought to the same temperature if more than a 5% change is occurring. Salinity should match that of the aquarium, or be dosed very slowly if altering the salinity. Aging and aerating saltwater (such as in a bucket with a powerhead or airstone) is recommended as good practice to allow the salts to fully ionize and the pH to stabilize.
Replacement water should be of the same source as the aquarium, whether it be reverse osmosis (RO), de-ionized (DI), distilled or from a 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.

Conservation
Almost all species kept in marine aquaria at this time are caught in the wild, although tank-raised specimens are becoming increasingly common as a viable alternative. Only a 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 discouraged but unfortunately 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, because it is created by coral polyps, takes many years if not centuries to form, and is a vital habitat for countless marine species; thus, commercial-scale harvesting of naturally-occurring live rock has been criticized by conservationists. Additionally, many animal 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 quickly and have markedly reduced lifespans compared to wild specimens. Often these specific environmental requirements cause improperly housed lifestock’s color and appearance to be 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. 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 within the domestic aquarium; the trade in frags (i.e. fragments) offers a fantastic opportunity for marine aquarists to obtain new and unique corals while limiting the impact 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. With the advent of large scale business operations focusing on breeding massive quantities of specimens, marine fishkeeping has become much more widespread than ever before. Perhaps the biggest turndown in marine fishkeeping is the initial setup cost. A 100 US gallon (400 L) reef tank full of coral and equipment can cost in excess of $2500 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.

http://en.wikipedia.org/wiki/Marine_aquarium