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. Fishkeepers use aquaria to keep fish, invertebrates, amphibians, marine mammals, turtles, and aquatic plants. The term combines the Latin root aqua, meaning water, with the suffix -arium, meaning "a place for relating to".
An aquarist owns fish or maintains an aquarium, typically constructed of glass or high strength acrylic 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. Specialized equipment maintains appropriate water quality and other characteristics suitable for the aquarium's residents.
istory and popularization
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. Introduction of
glass panes around the year 50 allowed Romans to replace one wall of marble
tanks, improving their view of the fish. In 1369, the Chinese Emperor, Hóngwu,
established a porcelain company that produced large porcelain tubs for
maintaining goldfish; over time, people produced tubs that approached the shape
of modern fish bowls. Leonhard Baldner, who wrote Vogel-, Fisch- und Tierbuch
(Bird, Fish, and Animal Book) in 1666, maintained weather loaches and
 Nineteenth century
Goldfish in a glass: portrait of Therese Krones, 1824
In 1836, soon after his invention of the Wardian case, Ward proposed to use
his tanks for tropical animals. In 1841 he did so, though only with aquatic
plants and toy fish. However, he soon housed real animals. 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.
An aquarium of the 1850s containing Vallisneria spiralis and coldwater fish
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 "aquatic vivarium" or
"aqua-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. Tank designs
and techniques for maintaining water quality were developed by Warington, later
cooperating with Gosse until his critical review of the tank water composition.
Edward Edwards developed these glass-fronted aquaria in his 1858 patent for a
"dark-water-chamber slope-back tank", with water slowly circulating to a
"What an Aquarium Should Be" - a humorous 1876 British engraving, apparently showing Thomas Huxley dreaming about sea creatures
Germans soon rivaled the British in their interest. In 1854, an anonymous author had two articles published about the saltwater aquaria of the United Kingdom: Die Gartenlaube (The Garden House) entitled Der Ocean auf dem Tische (The Ocean on the Table). 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
Pike in an aquarium c. 1908, at the Belle Isle Aquarium, Belle Isle Park
 Twentieth century
Circa 1908, the first mechanical aquarium air pump was invented, powered by running water, instead of electricity. The introduction of the air pump into the hobby is considered by several historians of the hobby to be a pivotal moment in its development.
Aquaria became more widely popular as houses had an electricity supply after World War I. Electricity allowed artificial lighting as well as aeration, filtration, and heating of the water. Initially, amateur aquarists kept native fish (with the exception of goldfish); the availability of exotic species from overseas further increased the popularity of the aquarium. Jugs made from a variety of materials were used to import fish from overseas, with 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 allowed fish to be successfully imported from distant regions. In the 1960s metal frames made marine aquaria almost impossible due to corrosion, but the development of tar and silicone sealant allowed the first all-glass aquaria made by Martin Horowitz in Los Angeles, CA. The frames remained, however, though purely for aesthetic reasons.
In the United States, as of 1996, aquarium keeping is the second-most popular
hobby after stamp collecting. In 1999 it was estimated that over nine
million U.S. households own an aquarium. Figures from the 2005/2006 APPMA
National Pet Owners Survey report that Americans own approximately 139 million
freshwater fish and 9.6 million saltwater fish. Estimates of the numbers
of fish kept in aquaria in Germany suggest 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.[citation
An 80 liter home aquarium
Most aquaria consist of glass panes bonded together by silicone, with plastic frames that are attached to the upper and lower edges for decoration. The glass aquarium is standard for sizes up to about 1,000 litres (260 US gal). However, glass as a material 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), and bow-front (the front side curves outwards). Fish bowls are generally either made of plastic or glass, and are either spherical or some other round configuration in shape.
The very first modern aquarium made of glass was developed in the 19th century by Robert Warrington. During the Victorian age it was common for glass aquaria to have slate or steel bottoms, which allowed the aquaria to be heated underneath with an open flame heat source. The aquaria in those days had the glass panels attached with metal frames and sealed with putty. These metal framed aquaria were still available on the market until the mid 1960s when the modern, silicone-sealed style replaced them. Acrylic tanks were not generally available to the public until the 1970s.
Although glass aquaria are usually preferred by aquarists over the acrylic ones because of their resistance to scratching and much more accessible price, they come with several disadvantages. Not only are they not as crack resistant as acrylic tanks but they are also nearly two times heavier than the latter. They also provide less insulation than acrylic aquaria and do not come in as many interesting shapes. Many aquarists or beginners who want to get fish as pets find it particularly onerous that many online suppliers will not ship glass aquaria because of the high potential for cracking and the high weight, which increases the cost of shipping. However, glass tanks are more convenient for other aquarists because unlike acrylic, glass does not yellow over time, and also because glass tanks do not need as much support as acrylic aquaria.
Even though the price is one of the main considerations for aquarists when deciding which of these two types of aquaria to purchase, when it comes to very large tanks the price difference tends to disappear.
Acrylic aquaria are also available and are the primary competitor with glass. Acrylic aquaria are stronger than glass, and much lighter. Acrylic-soluble cements are used to directly fuse acrylic together (as opposed to simply sealing the seam). Acrylic allows for the formation of unusual shapes, such as the hexagonal tank. Compared to glass, acrylics are easy to scratch; but unlike glass, it is possible to polish out scratches in acrylic.
Laminated glass is sometimes used, which combines the advantages of both glass and acrylic.
Large aquaria might instead 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 preventing contamination of the water by
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. In recent years, elaborate
custom-designed home aquariums costing hundreds of thousands of dollars have
become status symbols—according to The New York Times, "among people of means, a
dazzling aquarium is one of the last surefire ways to impress their
Desktop jellyfish tank using the kreisel design
A kreisel tank is a circular aquarium designed to hold delicate animals such as jellyfish. These aquariums provide slow, circular water flow with a bare minimum of interior hardware, to prevent delicate animals from becoming injured by pumps or the tank itself. Originally a German design (kreisel means spinning top), the tank has 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 circular tank has a circular, submerged lid. Pseudokreisels have a curved bottom surface and a flat top surface, similar to the shape of either a "U" or a semicircle. Stretch kreisels or Langmuir kreisels are a "double gyre" kreisel design, where the tank length is at least twice the height. Using two downwelling inlets on both sides of the tank lets gravity create two gyres in the tank. A single downwelling inlet may be used in the middle as well. The top of a stretch kreisel may be open or closed with a lid. There may also be screens about midway down the sides of the tank, or at the top on the sides. 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. It is now possible to start a jellyfish aquarium at home as easily as a regular fish tank.
Another popular setup is the biotope aquarium. A biotope aquarium is a
recreation of a specific natural environment. Some of the most popular biotopes
(to name only a few) are the Amazon river, Rio Negro River, Lake Malawi,
Lake Tanganyika, and Lake Victoria. The fish, plants, substrate, rocks, wood,
and any other component of the display should match that of the natural
environment. It can be a real challenge to recreate such environments and most
"true" biotopes will only have a few species of fish (if not only one) and
 Aquarium size and volume
Photo - silhouettes of people in foreground. One large fish with many smaller fish in background.
The 80-meter (260 ft) underwater tunnel in Aquarium Barcelona
An aquarium can range from a small glass bowl containing less than 1 litre
(2.1 US pt) of water to immense public aquaria that house entire ecosystems such
as kelp forests. Relatively large home aquaria resist rapid fluctuations of
temperature and pH, allowing for greater system stability.
World's smallest aquarium (10 ml volume) with fish (immature zebrafish)
Unfiltered bowl-shaped aquaria are now widely regarded as unsuitable for most fish. Advanced alternatives are now available. Aquariums should contain three forms of filtration: biological, mechanical and chemical to keep water conditions at suitable levels.
Reef aquaria under 100 litres (26 US gal) have a special place in the
aquarium hobby; these aquaria, termed nano reefs (when used in reefkeeping),
have a small water volume.
Lisbon Oceanarium designed by architect Peter Chermayeff
Practical limitations, most notably the weight of water (1 kilogram per litre (8.345 lb/U.S. gal)) and internal water pressure (requiring thick glass siding) of a large aquarium, restrict most home aquaria to a maximum of around 1 cubic metre in volume (1,000 kg or 2,200 lb). Some aquarists, however, have constructed aquaria of many thousands of litres.
Public aquariums designed for exhibition of large species or environments can
be dramatically larger than any home aquarium. The Georgia Aquarium, for
example, features an individual aquarium of 6,300,000 US gallons (23,800
Drawing of transparent 3-dimensional rectangle with two boxes and one cylinder above it and one longer, thin cylinder within it. Arrows point from the rectangle long cylinder to the top box, from the top box to the lower box, from the lower box to the other cylinder, from that cylinder back to itself, and from the cylinder to the rectangle.
Filtration system in a typical aquarium: (1) intake, (2) mechanical filtration, (3) chemical filtration, (4) biological filtration medium, (5) outflow to tank
The typical hobbyist aquarium includes a filtration system, an artificial lighting system, and a heater or chiller depending on the aquarium's inhabitants. Many aquaria incorporate a hood, to decrease evaporation and prevent fish from leaving the aquarium (and anything else from entering the aquarium). They also often hold lights.
Combined biological and mechanical aquarium filtration systems are common. These either convert ammonia to nitrate (removing nitrogen at the expense of aquatic plants), or to sometimes remove phosphate. Filter media can house microbes that mediate nitrification. Filtration systems are the most complex component of home aquaria.
Aquarium heaters combine a heating element with a thermostat, allowing the aquarist to regulate water temperature at a level above that of the surrounding air, whereas coolers and chillers (refrigeration devices) are for use anywhere, such as cold water aquaria, where the ambient room temperature is above the desired tank temperature. Thermometers used include glass alcohol thermometers, adhesive external plastic strip thermometers, and 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 bog-wood, 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.
An aquarium can be placed on an aquarium stand. Because of the weight of the
aquarium, a stand must be strong as well as level. A tank that is not level may
distort, leak, or crack. These are often built with cabinets to allow
storage, available in many styles to match room decor. Simple metal tank stands
are also available. Most aquaria should be placed on polystyrene to cushion
any irregularities on the underlying surface or the bottom of the tank
itself. However, some tanks have an underframe making this
 Aquarium maintenance
Photo of 50-foot-tall (15 m) yellow plants in water behind glass wall divided into sections.
A 335,000 U.S. gallon (1.3 million liter) aquarium at the Monterey Bay Aquarium in California, displaying a kelp forest ecosystem
Large volumes of water enable more stability in a tank by diluting effects from death or contamination events that push an aquarium away from equilibrium. The bigger the tank, the easier such a systemic shock is to absorb, because 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. For this reason, hobbyists often favor larger tanks, as they require less attention.
Several nutrient cycles are important in the aquarium. Dissolved oxygen enters the system at the surface water-air interface. Similarly, 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 enough to keep these other nutrient cycles in approximate equilibrium.
An aquarium must be maintained regularly to ensure that the fish are kept healthy. Daily maintenance consists of checking the fish for signs of stress and disease. Also, aquarists must make sure that the water has a good quality and it is not cloudy or foamy and the temperature of the water is appropriate for the particular species of fish that live in the aquarium.
Typical weekly maintenance includes changing around 10-20% of the water while
cleaning the gravel, or other substrate if the aquarium has one. A good habit is
to remove the water being replaced by "vacuuming" the gravel with suitable
implements, as this will eliminate uneaten foods and other residues that settle
on the substrate. Tap water is not considered to be safe for fish to live in
because it contains chemicals that harm the fish, so any tap water used must be
treated with a suitable water conditioner, such as a product which removes
chlorine and chloramine, and neutralises any heavy metals present. The water
parameters must be checked both in the tank and in the replacing water, to make
sure they are suitable for the species of fish kept.
 Water conditions
The solute content of water is perhaps the most important aspect of water conditions, as total dissolved solids and other constituents dramatically impact basic water chemistry, and therefore how organisms interact with their environment. Salt content, or salinity, is the most basic measure of water conditions. An aquarium may have freshwater (salinity below 500 parts per million), simulating a lake or river environment; brackish water (a salt level of 500 to 30,000 PPM), simulating environments lying between fresh and salt, such as estuaries; and salt water or seawater (a salt level of 30,000 to 40,000 PPM), simulating an ocean environment. Rarely, higher salt concentrations are maintained in specialized tanks for raising brine organisms.
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. Dissolved organic content and dissolved
gases content are also important factors.
Photo of water-filled glass tank containing with two green plants and pebbles on the bottom.
A typical home 10 US gallons (38 L) tropical freshwater aquarium
Home aquarists typically use tap water supplied through their local water supply network to fill their tanks. Straight tap water cannot be used in localities that pipe chlorinated water. 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. However, chloramine is now used more often and does 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 commercially available mixture of salts and other minerals.
Some aquarists modify water's alkalinity, hardness, or dissolved content of organics and gases, before adding it to their aquaria. This can be accomplished by additives, such as sodium bicarbonate, to raise pH. Some aquarists filter or purify their water through deionization or reverse osmosis prior to using it. In contrast, public aquaria with large water needs often locate themselves near a natural water source (such as a river, lake, or ocean) to reduce the level of treatment. Some hobbyists use an algae scrubber to filter the water naturally.
Water temperature determines the two most basic aquarium classifications: tropical versus cold water. Most fish and plant species tolerate only a limited temperature range; tropical aquaria, with an average temperature of about 25 °C (77 °F), are much more common. Cold water aquaria are for fish that are better suited to a cooler environment. More important than the range is consistency; most organisms are not accustomed to sudden changes in temperatures, which can cause shock and lead to disease. Water temperature can be regulated with a thermostat and heater (or cooler).
Water movement can also be important in simulating a natural ecosystem.
Aquarists may prefer anything from still water up to swift currents, depending
on the aquarium's inhabitants. Water movement can be controlled via 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 waste produced by an
aquarium's inhabitants. Fish, invertebrates, fungi, and some bacteria excrete
nitrogen waste in the form of ammonia (which converts 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.
 The process
A well-balanced tank contains organisms that are able to metabolize the waste products of other aquarium residents. This process is known in the aquarium hobby as the nitrogen cycle. Bacteria known as nitrifiers (genus Nitrosomonas) metabolize nitrogen waste. Nitrifying bacteria capture ammonia from the water and metabolize it to produce nitrite. Nitrite is toxic to fish in high concentrations. Another type of bacteria (genus Nitrospira) converts nitrite into nitrate, a less toxic substance. (Nitrobacter bacteria were previously believed to fill this role. 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.) However, commercial products sold as kits to "jump start" the nitrogen cycle often still contain Nitrobacter.
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 that decays
more slowly than ammonia-driven plankton already dissolved in the
 Maintaining the nitrogen cycle
What hobbyists call the nitrogen cycle is only a portion of the complete 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. The aquarium keeper must remove water once nitrate concentrations grow, or remove plants which have grown from the nitrates.
Hobbyist aquaria often do not have sufficient bacteria populations to adequately denitrify waste. This problem is most often addressed through two filtration solutions: Activated carbon filters absorb nitrogen compounds and other toxins, while biological filters provide a medium designed to enhance bacterial colonization. Activated carbon and other substances, such as ammonia absorbing resins, stop working when their pores fill, so these components have to be replaced regularly.
New aquaria often have problems associated with the nitrogen cycle due to insufficient beneficial bacteria. Therefore fresh water has to be matured before stocking them with fish. There are three basic approaches to this: the "fishless cycle", the "silent cycle" and "slow growth".
In a fishless cycle, small amounts of ammonia are added to an unpopulated tank to feed the bacteria. 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, the plants can consume nitrogenous waste so efficiently that ammonia and nitrite level spikes seen in more traditional cycling methods are greatly reduced or disappear. "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. This method is usually done with a small starter population of hardier fish which can survive the ammonia and nitrite spikes, whether they are intended to be permanent residents or to be traded out later for the desired occupants.
The largest bacterial populations are found in the filter, where is high
water flow and plentiful surface available for their growth, so effective and
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
half of the filter media during each service, or using two filters, only one of
which is cleaned at a time.
 Biological loading
Photo displaying plants, small fish, and what appear to be tipped-over orange vases
Small aquarium (19 liter) with Paracheirodon innesi, Trigonostigma heteromorpha, and Hemigrammus erythrozonus
Biological load is a measure of the burden placed on the aquarium ecosystem
by its inhabitants. High biological loading presents a more complicated tank
ecology, which in turn means that equilibrium is easier to upset. Several
fundamental constraints on biological loading depend on aquarium size. The
water's surface area limits oxygen intake. The bacteria population depends on
the physical space they have available to colonize. Physically, only a limited
size and number of plants and animals can fit into an aquarium while still
providing room for movement. Biologically, biological loading refers to the rate
of biological decay in proportion to tank volume.
 Calculating capacity
Limiting factors include the oxygen availability and filtration processing. Aquarists have rules of thumb to estimate the number of fish that can be kept in an aquarium. The examples below are for small freshwater fish; larger freshwater fishes and most marine fishes need much more generous allowances.
* 3 cm of adult fish length per 4 litres of water (i.e., a
6 cm-long fish would need about 8 litres of water).
* 1 cm of adult fish length per 30 square centimetres of surface area.
* 1 inch of adult fish length per US gallon of water.
* 1 inch of adult 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, surface area of plant life, and so on. Establishing
maximum capacity is often a matter of slowly adding fish and monitoring water
quality over time, following a trial and error approach.
 Other factors affecting capacity
One variable is differences between fish. Smaller fish consume more oxygen per gram of body weight than larger fish. Labyrinth fish can breathe atmospheric oxygen and do not need as much surface area (however, some of these fish are territorial, and do 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 matters. Some aquarists claim that a deeper aquarium holds no more fish than a shallower aquarium with 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 waste decomposition rates.
Waste density is another variable. Decomposition in solution consumes oxygen. Oxygen dissolves less readily in warmer water; this is a double-edged sword since warmer temperatures make fish more active, so they consume more oxygen.
In addition to bioload/chemical considerations, aquarists also consider the
mutual compatibility of the fish. For instance, predatory fish are usually not
kept with small, passive species, and territorial fish are often unsuitable
tankmates for shoaling species. Furthermore, fish tend to fare better if given
tanks conducive to their size. That is, large fish need large tanks and small
fish can do well in smaller tanks. Lastly, the tank can become overcrowded
without being overstocked. In other words, the aquarium can be suitable with
regard to filtration capacity, oxygen load, and water, yet still be so crowded
that the inhabitants are uncomfortable.
 Aquarium classifications
Photo showing a tank filled with water and multiple aquatic plants.
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 only a single small fish, to the huge public aquaria that can simulate entire marine ecosystems.
One way to classify aquaria is by salinity. Freshwater aquaria are the most popular due to their lower cost.More expensive and complex equipment is required to set up and maintain a marine aquaria. Marine aquaria frequently feature a diverse range of invertebrates in addition to species of fish. 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 mangrove swamps and estuaries. Subtypes exist within these types, such as the reef aquarium, a typically smaller marine aquarium that houses coral.
Another classification is by temperature range. Many aquarists choose a
tropical aquarium because tropical fish tend to be more colorful. However,
the coldwater aquarium is also popular, which is mainly restricted to goldfish,
but can include fish from temperate areas worldwide and native fish
Photo of water, coral, and fish behind a glass wall.
A saltwater aquarium
Aquaria may be grouped by their species selection. The community tank is the most common today, where several non-aggressive species live peacefully. In these aquaria, the fish, invertebrates, and plants probably do not originate from the same geographic region, but tolerate similar water conditions. Aggressive tanks, in contrast, house a limited number of species that can be aggressive toward other fish, or are able to withstand aggression well. Most marine tanks and tanks housing cichlids have to take the aggressiveness of the desired species into account when stocking. Specimen tanks usually only house one fish species, along with plants, perhaps ones found in the fishes' natural environment and decorations simulating a natural ecosystem. This type is useful for fish that cannot coexist 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.
Ecotype, ecotope, or biotope aquaria is another type based on species
selection. In it, an aquarist attempts to simulate a specific natural ecosystem,
assembling fish, invertebrate species, plants, decorations and water conditions
all found in that ecosystem. These biotope aquaria are the most sophisticated
hobby aquaria; public aquaria use this approach whenever possible. This approach
best simulates the experience of observing in the wild. It typically serves as
the healthiest possible artificial environment for the tank's
 Public aquaria
Photo looking upward through 15 feet (4.6 m)-diameter glass tube into a fish-filled aquarium
Tunnel at the world's largest aquarium, Georgia Aquarium, USA
Main article: Public aquarium
Most public aquarium facilities feature a number of smaller aquaria, as well
those too large for home aquarists. The largest tanks hold millions of gallons
of water and can house large species, including sharks or beluga whales.
Dolphinaria are specifically for dolphins. Aquatic and semiaquatic animals,
including otters and penguins, may also be kept by public aquaria. Public
aquaria may also be included in larger establishments such as a marine mammal
park or a marine park.
 Virtual aquariums
A virtual aquarium is a computer program which uses 3D graphics to reproduce an aquarium on a personal computer. The swimming fish are rendered in real time, while the background of the tank is usually static. Objects on the floor of the tank may be mapped in simple planes so that the fish may appear to swim both in front and behind them, but a relatively simple 3D map of the general shape of such objects may be used to allow the light and ripples on the surface of the water to cast realistic shadows. Bubbles and water noises are common for virtual aquariums, which are often used as screensavers.
The number of each type of fish can usually be selected, often including other animals like starfish, jellyfish, seahorses, and even sea turtles. Most companies that produce virtual aquarium software also offer other types of fish for sale via Internet download. Other objects found in an aquarium can also be added and rearranged on some software, like treasure chests and giant clams that open and close with air bubbles, or a bobbing diver. There are also usually features that allow the user to tap on the glass or put food in the top, both of which the fish will react to. Some also have the ability to allow the user to edit fish and other objects to create new varieties.