Aquarium

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A freshwater aquarium with plants and tropical fish.An aquarium (plural aquariums or aquaria) is a vivarium consisting of at least one transparent side in which water-dwelling plants or animals are kept. Aquaria are primarily used for fishkeeping, although invertebrates, amphibians, marine mammals, and aquatic plants are also housed in aquaria. The term combines the Latin root aqua, meaning water, with the suffix -arium, meaning "a place for relating to".

The aquarium has a long history and maintaining an aquarium has become immensely popular worldwide; a person who owns or maintains an aquarium is known as an aquarist. Aquaria can come in a variety of materials, shapes, and sizes. They are typically constructed of glass or high-strength plastic. Cuboid aquaria are also known as fish tanks or simply tanks, while bowl-shaped aquaria are also known as fish bowls. Size can range from a small glass bowl to immense public aquaria. A number of components are used to maintain appropriate water quality and characteristics suitable for the aquarium's residents. There are many types of aquaria, classified by the organisms maintained or the type of environment that is mimicked.

History and popularization
Fishkeeping in artificial environments has existed for centuries. In the Roman Empire, the first fish to be brought indoors was the sea barbel, which was kept under guest beds in small tanks made of marble. With the introduction of glass panes around the year 50, the Romans replaced one wall of the marble tank, improving their view of the fish. In 1369, the Chinese Emperor, Hongwu, established a porcelain company that produced large porcelain tubs for maintaining goldfish; over time, these tubs were produced more and more like modern fish bowls. Leonhard Baldner, who wrote Vogel-, Fisch- und Tierbuch (Bird, Fish, and Animal Book) in 1666, maintained weather loaches and newts.

In 1836, soon after his invention of the Wardian case, Ward proposed to use his tanks for tropical animals, and in 1841 he did so, though only with aquatic plants and toy fish. However, real animals were soon housed. In 1838, Félix Dujardin noted owning a saltwater aquarium, though he did not use the term. In 1846, Anna Thynne maintained stony corals and seaweed for almost three years, and was credited as the creator of the first balanced marine aquarium in London. At about the same time, Robert Warington experimented with a 13-gallon container, which contained goldfish, eelgrass, and snails, creating one of the first stable aquaria; he published his findings in 1850 in the Chemical Society's journal.


Pike in an aquarium c. 1908, at the Detroit Aquarium, Belle Isle Park.The keeping of fish in an aquarium became a popular hobby and spread quickly. In the United Kingdom, it became popular after ornate aquaria in cast iron frames were featured at the Great Exhibition of 1851. In 1853, the first large public aquarium opened in the London Zoo and came to be known as the Fish House. Philip Henry Gosse was the first person to actually use the word "aquarium", opting for this term (instead of "vivarium") in 1854 in his book The Aquarium: An Unveiling of the Wonders of the Deep Sea. In this book, Gosse primarily discussed saltwater aquaria. In the 1850s, the aquarium became a fad in the United Kingdom.


Germans soon rivaled the British in their interest. In 1854, two articles about the saltwater aquaria of the United Kingdom were published in Die Gartenlaube (The Garden House) entitled Der Ocean auf dem Tische (The Ocean on the Table) by an anonymous author. However, in 1856, Der See im Glase (The Lake in a Glass) was published, discussing freshwater aquaria, which were much easier to maintain in landlocked areas. During the 1870s, some of the first aquarist societies were appearing in Germany. The United States soon followed. Published in 1858, Henry D. Butler's The Family Aquarium was one of the first books written in the United States solely about the aquarium. According to the July issue of The North American Review of the same year, William Stimson may have owned some of the first functional aquaria, and had as many as seven or eight. The first aquarist society in the United States was founded in New York City in 1893, followed by others. The New York Aquarium Journal, first published in October 1876, is considered to be the world's first aquarium magazine.

In the Victorian era in the United Kingdom, a common design for the home aquarium was a glass front with the other sides made of wood (made watertight with a pitch coating). The bottom would be made of slate and heated from below. More advanced systems soon began to be introduced, along with tanks of glass in metal frames. During the latter half of the 19th century, a variety of aquarium designs were explored, such as hanging the aquarium on a wall, mounting it as part of a window, or even combining it with a birdcage.

Aquaria became more widely popular as houses became almost universally electrified after World War I. With electricity great improvements were made in aquarium technology, allowing artificial lighting as well as aeration, filtration, and heating of the water. Initially, amateur aquarists kept native fish (with the exception of goldfish); the entrance of exotic species from overseas also allowed aquaria to become even more popular. Jugs made from a variety of materials were initially used to import fish from overseas, including innovations such as using a bicycle foot pump for aeration. Plastic shipping bags were introduced in the 1950s, making it easier to ship fish. The eventual availability of air freight, which allowed a much wider variety of fish, especially marine, to be successfully imported from distant regions of origin, consequently attracted new hobbyists. In the 1960s, a great breakthrough was reached. Metal frames made marine aquaria almost impossible due to corrosion, but the development of silicone sealant allowed the first all-glass aquaria. The frames remained, however, though purely for aesthetic reasons.

Aquarium keeping is now a popular hobby around the world, with about 60 million aquarists worldwide.[citation needed] In the United States, aquarium keeping is the second-most popular hobby after stamp collecting.[22] The number of fish kept in aquaria vary by country. In 1999 it was estimated that over nine million U.S. households own an aquarium. In 2005, it was estimated that 139 million freshwater fish and 9 million saltwater fish were kept in aquaria in the U.S. Similarly, estimates of the numbers of fish kept in aquaria in Germany number at least 36 million. The hobby has the strongest following in Europe, Asia, and North America. In the United States, 40 percent of aquarists maintain two or more tanks at any one time.[citation needed]


Design

Materials
Most aquaria consist of simple glass panes bonded together by silicone. Usually plastic frames are attached to the upper and lower edges for decoration. Price, availability, and reliability make the glass aquarium an industry standard for sizes up to about 1000 litres (250 gal). However, glass is brittle and has very little give before fracturing, though generally the sealant fails first. Aquaria come in a variety of shapes such as cuboid, hexagonal, angled to fit in a corner (L-shaped), bow-front (the front side curves outwards), and more. Fish bowls are generally either plastic or glass, either spherical or some other round configuration.

Acrylic aquaria are also available and are the primary competitor with glass. Acrylics are stronger than glass, and much lighter. Acrylic-soluble cements are used to directly fuse acrylic together (as opposed to simply sealing the seam).[25] Acrylic allows for the formation of unusual shapes, such as hexagonal. Compared to glass, acrylics are easy to scratch; care must be taken with organisms with shells and teeth.

Laminated glass might be used, which combines the advantages of both glass and acrylic.

Large aquaria might use stronger materials such as fiberglass-reinforced plastics. However, this material is not transparent. Reinforced concrete is used for aquaria where weight and space are not factors. Concrete must be coated with a waterproof layer to prevent the water from breaking down the concrete as well as prevent contamination from the concrete.


Styles
Aquaria can come in more creative flavors. Aquariums have been fashioned into coffee tables, sinks, and even toilets. Another such example is the Macquarium, an aquarium made from the shell of an Apple Macintosh computer.[27]

A kreisel tank is a circular aquarium designed to hold delicate animals such as jellyfish. These aquariums provide slow, circular water flow with a lack of physical objects.[28] Originally a German design (kreisel means spinning top), the tank is designed to have no sharp corners, and keeps the housed animals away from the plumbing. Water moving into the tank gives a gentle flow that keeps the inhabitants suspended, and water leaving the tank is covered by a delicate screen that prevents the inhabitants from getting stuck. There are several types of kreisel tanks. In a true kreisel, a perfectly circular tank has a circular, submerged, lid. Pseudokreisels, have a curved bottom surface and a top surface flat, similar to the shape of either a "U" or a semicircle.[29] It is possible to combine these designs; a circular shaped tank is used without a lid or cover, and the surface of the water acts as the continuation of circular flow.


Aquarium size and volume

A large aquarium at the Osaka Aquarium houses a whale shark.An aquarium can range from a small glass bowl containing less than a litre (34 fl.oz.) of water to immense public aquaria which can house entire ecosystems such as kelp forests. Larger aquaria are typically recommended to hobbyists due to their resistance to rapid fluctuations of temperature and pH, allowing for greater system stability.[26]

Aquaria kept in homes by hobbyists can be as small as 11 litres (3 gal); this size is widely considered the smallest practical system with filtration and other basic systems.[citation needed] On the other hand, reef aquaria under 100 litres (20 gal) earn a special place in the aquarium hobby; these aquaria, termed nano reefs, are known to be more difficult due to their small water volume.[citation needed] Practical limitations, most notably the weight (One litre of fresh water has a mass of 1 kilogram (8.3 lb gal-1), and salt water is even denser) and internal water pressure (requiring thick, strong glass siding) of a large aquarium, keep most home aquaria to a maximum of around 1 cubic metre in volume (1,000 kg or 2,200 lb). Indeed, larger aquariums can even threaten the floor beneath the aquarium.[26] Some dedicated aquarists, however, have been known to construct custom aquaria of up to many thousands of litres, at great effort and expense.[30][31]

Aquaria within public aquariums designed for exhibition of large species or environments can be dramatically larger than any home aquarium. The Shedd Aquarium in Chicago, for example, features an individual aquarium of 7.7 million litres (2 million gallons).


Components
Filtration system in a typical aquarium: (1)Intake.(2) Mechanical filtration.(3) Chemical filtration. Biological filtration medium. Outflow to tank.The typical hobbyist aquarium includes a filtration system, an artificial lighting system, and a heater or chiller depending on the inhabitants of the aquarium. Many aquaria incorporate a hood, which prevents evaporation and protects fish from leaving the aquarium (or anything else from entering the aquarium). They also often hold lights. Hoods can be opened and closed easily for access into the aquarium.

Combined biological and mechanical aquarium filtration systems are commonly used; these are designed to either convert ammonia to nitrate or remove it or sometimes remove phosphate from water, removing them being at the expense of aquatic plants. Particulates incorporated into the filter can provide energy for microbes and sponges that do these things. Filtration systems are often the most complex component of home aquaria, and various designs and combinations are used.

Aquarium heaters combine a heating element with a thermostat, allowing an aquarist to regulate water temperature at a level above that of the surrounding air, whereas coolers and chillers (refrigeration devices) are for use in cold water aquaria, or anywhere the ambient room temperature is above the desired tank temperature. A variety of different thermometers are used, such as the glass alcohol thermometers, adhesive external plastic strip thermometers, and even battery-powered LCD thermometers. In addition, some aquarists use air pumps attached to airstones or water pumps to increase water circulation and supply adequate gas exchange at the water surface. Wave-making devices have also been constructed to provide wave action.

An aquarium's physical characteristics form another aspect of aquarium design. Size, lighting conditions, density of floating and rooted plants, placement of bogwood, creation of caves or overhangs, type of substrate, and other factors (including an aquarium's positioning within a room) can all affect the behavior and survival of tank inhabitants. The combined function of these elements is to maintain appropriate water quality and characteristics suitable for the aquarium's residents.

An aquarium is often also placed on a specially-made aquarium stand. Because of the weight of an aquarium, they must be strong as well as level. A tank that is not level may distort, leak, or crack.[26] These are often built like cabinets to allow storage, available in many styles so it can match room decor. Simple metal tank stands are also available.[26] Some sources say that polystyrene should be placed under the aquarium as a safety precaution.[26]


Aquarium maintenance

A 335,000 U.S. gallon (1.3 million litre) aquarium at the Monterey Bay Aquarium in California displaying a simulated kelp forest ecosystemIdeal aquarium ecology reproduces the balance found in nature in the closed system of an aquarium. In practice it is virtually impossible to maintain a perfect balance. As an example, a balanced predator-prey relationship is nearly impossible to maintain in even the largest of aquaria. Typically an aquarium keeper must take steps to maintain balance in the small ecosystem contained in his aquarium.

Approximate balance is facilitated by large volumes of water. Any event that perturbs the system pushes an aquarium away from equilibrium; the more water that is contained in a tank, the easier such a systemic shock is to absorb, as the effects of that event are diluted. For example, the death of the only fish in a three U.S. gallon tank (11 L) causes dramatic changes in the system, while the death of that same fish in a 100 U.S. gallon (400 L) tank with many other fish in it represents only a minor change in the balance of the tank. For this reason, hobbyists often favor larger tanks when possible, as they are more stable systems requiring less intensive attention to the maintenance of equilibrium.

There are a variety of nutrient cycles that are important in the aquarium. Dissolved oxygen enters the system at the surface water-air interface or through the actions of an air pump. Carbon dioxide escapes the system into the air. The phosphate cycle is an important, although often overlooked, nutrient cycle. Sulfur, iron, and micronutrients also cycle through the system, entering as food and exiting as waste. Appropriate handling of the nitrogen cycle, along with supplying an adequately balanced food supply and considered biological loading, is usually enough to keep these other nutrient cycles in approximate equilibrium.


Water conditions
The solute content of water is perhaps the most important aspect of water conditions, as total dissolved solids and other constituents can dramatically impact basic water chemistry, and therefore how organisms are able to interact with their environment. Salt content, or salinity, is the most basic classification of water conditions. An aquarium may have fresh water (salinity below 0.5 PPT), simulating a lake or river environment; brackish water (a salt level of 0.5 to 30 PPT), simulating environments lying between fresh and salt, such as estuaries; and salt water or sea water (a salt level of 30 to 40 PPT), simulating an ocean or sea environment. Rarely, even higher salt concentrations are maintained in specialized tanks for raising brine organisms.

Several other water characteristics result from dissolved contents of the water, and are important to the proper simulation of natural environments. Saltwater is typically alkaline, while the pH (alkalinity or acidicity) of fresh water varies more. Hardness measures overall dissolved mineral content; hard or soft water may be preferred. Hard water is usually alkaline, while soft water is usually neutral to acidic.[33] Dissolved organic content and dissolved gases content are also important factors.


An average home 10 gallon tropical Freshwater Aquarium.Home aquarists typically use modified tap water supplied through their local water supply network to fill their tanks. Because of the chlorine used to disinfect drinking water supplies for human consumption, straight tap water cannot be used. In the past, it was possible to "condition" the water by simply letting the water stand for a day or two, which allows the chlorine time to dissipate.[33] However, chloramine is now used more often as it is much stabler and will not leave the water as readily. Additives formulated to remove chlorine or chloramine are often all that is needed to make the water ready for aquarium use. Brackish or saltwater aquaria require the addition of a mixture of salts and other minerals, which are commercially available for this purpose.

More sophisticated aquarists may make other modifications to their base water source to modify the water's alkalinity, hardness, or dissolved content of organics and gases, before adding it to their aquaria. This can be accomplished by a range of different additives, such as sodium bicarbonate to raise pH.[33] Some aquarists will even filter or purify their water prior to adding it to their aquarium. There are two processes used for that: deionization or reverse osmosis. In contrast, public aquaria with large water needs often locate themselves near a natural water source (such as a river, lake, or ocean) in order to have easy access to a large volume of water that does not require much further treatment.

The temperature of the water forms the basis of one of the two most basic aquarium classifications: tropical vs. cold water. Most fish and plant species tolerate only a limited range of water temperatures: Tropical or warm water aquaria, with an average temperature of about 25 °C (77 °F), are much more common, and tropical fish are among the most popular aquarium denizens. Cold water aquaria are those with temperatures below what would be considered tropical; a variety of fish are better suited to this cooler environment. More importantly than the temperature range itself is the consistency in temperature; most organisms are not accustomed to sudden changes in temperatures, which could cause shock and lead to disease.[33] Water temperature can be regulated with a combined thermometer and heater unit (or, more rarely, with a cooling unit).

Water movement can also be important in accurately simulating a natural ecosystem. Aquarists may prefer anything from still water up to swift simulated currents in an aquarium, depending on the conditions best suited for the aquarium's inhabitants. Water movement can be controlled through the use of aeration from air pumps, powerheads, and careful design of internal water flow (such as location of filtration system points of inflow and outflow).


Nitrogen cycle

The nitrogen cycle in an aquarium.Main article: Nitrogen cycle
Of primary concern to the aquarist is management of the biological waste produced by an aquarium's inhabitants. Fish, invertebrates, fungi, and some bacteria excrete nitrogen waste in the form of ammonia (which will convert to ammonium, in acidic water) and must then pass through the nitrogen cycle. Ammonia is also produced through the decomposition of plant and animal matter, including fecal matter and other detritus. Nitrogen waste products become toxic to fish and other aquarium inhabitants at high concentrations.[33]


The process
A well-balanced tank contains organisms that are able to metabolize the waste products of other aquarium residents. The nitrogen waste produced in a tank is metabolized in aquaria by a type of bacteria known as nitrifiers (genus Nitrosomonas). Nitrifying bacteria capture ammonia from the water and metabolize it to produce nitrite. Nitrite is also highly toxic to fish in high concentrations. Another type of bacteria, genus Nitrospira, converts nitrite into nitrate, a less toxic substance to aquarium inhabitants. (Nitrobacter bacteria were previously believed to fill this role, and continue to be found in commercially available products sold as kits to "jump start" the nitrogen cycle in an aquarium. While biologically they could theoretically fill the same niche as Nitrospira, it has recently been found that Nitrobacter are not present in detectable levels in established aquaria, while Nitrospira are plentiful.) This process is known in the aquarium hobby as the nitrogen cycle.

In addition to bacteria, aquatic plants also eliminate nitrogen waste by metabolizing ammonia and nitrate. When plants metabolize nitrogen compounds, they remove nitrogen from the water by using it to build biomass. However, this is only temporary, as the plants release nitrogen back into the water when older leaves die off and decompose.


Maintaining the nitrogen cycle
Although informally called the nitrogen cycle by hobbyists, it is in fact only a portion of a true cycle: nitrogen must be added to the system (usually through food provided to the tank inhabitants), and nitrates accumulate in the water at the end of the process, or become bound in the biomass of plants. This accumulation of nitrates in home aquaria requires the aquarium keeper to remove water that is high in nitrates, or remove plants which have grown from the nitrates.

Aquaria kept by hobbyists often do not have the requisite populations of bacteria needed to detoxify nitrogen waste from tank inhabitants. This problem is most often addressed through two filtration solutions: Activated carbon filters absorb nitrogen compounds and other toxins from the water, while biological filters provide a medium specially designed for colonization by the desired nitrifying bacteria. Activated carbon and other substances, such as ammonia absorbing resines, will stop working when their pores get full, so these components have to be replaced with fresh stocks constantly.

New aquaria often have problems associated with the nitrogen cycle due to insufficient number of beneficial bacteria, known as the "New Tank Syndrome". Therefore new tanks have to be "matured" before stocking them with fish. There are three basic approaches to this: the fishless cycle the silent cycle and slow growth.

No fish are kept in a tank undergoing a fishless cycle. Instead, small amounts of ammonia are added to the tank to feed the bacteria being cultured. During this process, ammonia, nitrite, and nitrate levels are tested to monitor progress. The silent cycle is basically nothing more than densely stocking the aquarium with fast-growing aquatic plants and relying on them to consume the nitrogen, allowing the necessary bacterial populations time to develop. According to anecdotal reports of aquarists specializing in planted tanks, the plants can consume nitrogenous waste so efficiently that the spikes in ammonia and nitrite levels normally seen in more traditional cycling methods are greatly reduced, if they are detectable at all. More commonly slow growth entails slowly increasing the population of fish over a period of 6 to 8 weeks, giving bacteria colonies time to grow and stabilize with the increase in fish waste.

The largest bacterial populations are found in the filter; efficient filtration is vital. Sometimes, a vigorous cleaning of the filter is enough to seriously disturb the biological balance of an aquarium. Therefore, it is recommended to rinse mechanical filters in an outside bucket of aquarium water to dislodge organic materials that contribute to nitrate problems, while preserving bacteria populations. Another safe practice consists of cleaning only one half of the filter media every time the filter or filters are serviced.


Biological loading

19 Litre AquariumBiological loading is a measure of the burden placed on the aquarium ecosystem by its living inhabitants. High biological loading in an aquarium represents a more complicated tank ecology, which in turn means that equilibrium is easier to perturb. In addition, there are several fundamental constraints on biological loading based on the size of an aquarium. The surface area of water exposed to air limits dissolved oxygen intake by the tank. The capacity of nitrifying bacteria is limited by the physical space they have available to colonize. Physically, only a limited size and number of plants and animals can be fit into an aquarium while still providing room for movement.


Calculating aquarium capacity
An aquarium can only support a certain number of fish. Limiting factors include the availability of oxygen in the water and the rate at which the filter can process waste. Aquarists have developed a number of rules of thumb to allow them to estimate the number of fishes that can be kept in a given aquarium; the examples below are for small freshwater fish, larger freshwater fishes and most marine fishes need much more generous allowances.

3 cm of fish length per 4 litres of water (i.e., a 6 cm-long fish would need about 8 litres of water).
1 cm of fish length per 30 square centimetres of surface area.
1 inch of fish length per gallon of water.
1 inch of fish length per 12 square inches of surface area.
Experienced aquarists warn against applying these rules too strictly because they do not consider other important issues such as growth rate, activity level, social behaviour, and so on. To some degree, establishing the maximum loading capacity of an aquarium depends upon slowly adding fish and monitoring water quality over time, essentially a trial and error approach.


Factors affecting capacity
Though many conventional methods of calculating the capacity of aquarium is based on volume and pure length of fish, there are other variables. One variable is differences between fish. Smaller fish consume more oxygen per gram of body weight than larger fish. Labyrinth fish, having the capability to breathe atmospheric oxygen, are noted for not needing as much surface area (however, some of these fish are territorial, and may not appreciate crowding). Barbs also require more surface area than tetras of comparable size.

Oxygen exchange at the surface is an important constraint, and thus the surface area of the aquarium. Some aquarists go so far as to say that a deeper aquarium with more volume holds no more fish than a shallower aquarium of the same surface area. The capacity can be improved by surface movement and water circulation such as through aeration, which not only improves oxygen exchange, but also the decomposition of waste materials.

The presence of waste materials presents itself as a variable as well. Decomposition is an oxygen-consuming process, therefore the more decaying matter there is, the less oxygen as well. Oxygen dissolves less readily in warmer water; this is a double-edged sword as warmer temperatures make more active fish, which in turn consume even more oxygen. Stress due to temperature changes is especially obvious in coldwater aquaria where the temperature may swing from low temperatures to high temperatures on hotter days.


Aquarium classifications

A planted freshwater aquarium.From the outdoor ponds and glass jars of antiquity, modern aquaria have evolved into a wide range of specialized systems. Individual aquaria can vary in size from a small bowl large enough for a single small fish, to the huge public aquaria that can simulate entire marine ecosystems. A variety of different aquarium types exist; for the most part, many of these classifications are based on the environment the aquarium intends to mimic.

One of the most basic ways to classify aquaria is their salinity. Freshwater aquaria are the most popular kind of aquarium due to their lower cost and ease of maintenance. Marine aquaria are generally require more complex equipment to set up and maintain than freshwater aquaria. Along with fish species, marine aquaria frequently feature a diverse range of invertebrates. Brackish water aquaria combine elements of both marine and freshwater fishkeeping. Fish kept in brackish water aquaria generally come from habitats with varying salinity, such as mangroves and estuaries. Certain subtypes of aquaria also exist within these types, such as the reef aquarium, a type of marine aquarium that houses coral.

Another method to classify aquaria is their temperature range. Most aquarists maintain a tropical aquarium as these fish tend to be more colorful. However, the coldwater aquarium is also popular, which often includes fish such as goldfish.


A saltwater aquarium.Aquaria may be grouped by their species selection. The community tank is the most common type of aquarium kept today, where several non-aggressive species are housed peacefully together. Aggressive tanks, in contrast, house a limited number of species that can be aggressive toward other fish, or are able to withstand aggression well. Species or specimen tanks usually only house one fish species, along with plants, perhaps found in the fishes' natural environment and decorations simulating a true ecosystem. This type is useful for fish that simply cannot be housed safely with other fish, such as the electric eel, as an extreme example. Some tanks of this sort are used simply to house adults for breeding. In these aquaria, the aquarium fish, invertebrates, and plants may or may not originate from the same geographic region, but generally tolerate similar water conditions.

Ecotype, ecotope, or biotope aquaria is another type based on species selection. In it, an aquarist attempts to simulate a specific ecosystem found in the natural world, bringing together fish, invertebrate species, and plants found only in that ecosystem in a tank with water conditions and decorations designed to simulate their natural environment. These ecotype aquaria might be considered the most sophisticated hobby aquaria; indeed, reputable public aquaria all use this approach in their exhibits whenever possible. This approach best simulates the experience of observing an aquarium's inhabitants in the wild, and also usually serves as the healthiest possible artificial environment for the tank's occupants.


Public aquaria

One of the large aquaria at the Georgia Aquarium, USA.Main article: Public aquarium
Public aquariums are facilities open to the public for viewing of aquatic species in aquaria. Most public aquariums feature a number of smaller aquaria, as well those greater in size than could be kept by home aquarists. The largest tanks hold millions of U.S. gallons of water and can house large species, including sharks or beluga whales. Dolphinariums are aquaria specifically for housing dolphins. Aquatic and semiaquatic animals, including otters and penguins, may also be kept by public aquariums. Public aquariums may also be included in larger establishments such as a marine mammal park or a marine park.