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 also requires more equipment than freshwater systems, and the aquarium inhabitants are often more expensive to acquire.
Main article: 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. 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.
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 sleep, 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 invertbrates, 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: 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 2 factors to consider. a) Wattage/Power b) color temperature. Depending on the type of lighting i.e. flourescents, MH etc, the wattage of light emitted varies: 18 W, 36 W, 150 W 250 W etc. Wattage is equivalent to power and determines how brightly the light will shine. The deeper and bigger the aquarium the higher wattage you'd need. Color Temperature refers to the spectrum of light being emitted by the lamp and they're measured in degree Kelvin (K). Again in lay mans' terms, this refers to the type of light. Light from the sun has a color temperature of approximately 6500 K and they're perfect for growing plants in refugiums. 10,000 K light appears white and will give good coloration to fishes and corals. Moving up the spectrum there is 14,000 and 20,000 K bulbs that produce a bluish tint that mimics the lighting conditions under the sea creating a great environment for livestock in the aquarium.
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 24-28 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 aquarium heater. In some areas ambient temperature is greater than the desired temperature and refridgeration devices, known as chillers are used to cool aquarium water.
Marine aquarists commonly test the water in the aquarium for a variety of chemical indicators of water quality, these include:
Specific gravity, a measure ofwater denisty, 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 28-32 PPT. Salinity is directly related to specific gravity and both can be tested with a inexpensive hydrometer or a refractometer.
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. 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 (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 40 ppm 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 are a staple of good saltwater maintenance. (Although controversial, larger (approx 200 Gallons) 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. Supplements are sometimes needed to add calcium, alkalinity, etc.) 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. 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 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 although the number of captive raised species are increasing everyday. 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 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 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 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 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 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.
Most of this information was derived from Wikipedia