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. temperate or coldwater aquaria are for fish that are better suited to a cooler environment. temperature consistency is more important than range. 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 main article: nitrogen cycle the nitrogen cycle in an aquarium 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 water) and must then either pass through the nitrogen cycle or be removed by passing through zeolite. 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. in the wild, the vast amount of water surrounding the fish dilutes ammonia and other waste materials. when fish are put into an aquarium, waste can quickly reach toxic concentrations in the enclosed environment unless the tank is cycled to remove waste. the process a well-balanced tank contains organisms that are able to metabolize the waste products of other aquarium residents, recreating a portion of 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. 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 water. some hobbyists also use "anoxic filtration," which relies on bacteria that live in low-oxygen environments. maintaining the nitrogen cycle live plants in an aquarium utilize the final product in the nitrogen cycle of nitrate as fertilizer, helping the nitrate levels stay minimal. this 60-litre (13 imp gal; 16 us gal) aquarium contains anubias barteri and echinodorus bleheri. a heater and small filter are in the background. the nitrogen cycle in an aquarium 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 there 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 load a very heavily stocked 19-liter aquarium containing paracheirodon innesi, trigonostigma heteromorpha, and hemigrammus erythrozonus the biological load, or bioload, 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. adding plants to an aquarium will sometimes help greatly with taking up fish waste as plant nutrients. although an aquarium can be overloaded with fish, an excess of plants is unlikely to cause harm. decaying plant material, such as decaying plant leaves, can add these nutrients back into the aquarium if not promptly removed. the bioload is processed by the aquarium's biofilter filtration system. 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, filtration capacity, total biomass of plant life, and so on. it is better to apply the overall mass and size of a fish per gallon of water, than simply the length. this is because fish of different sizes produce quite differing amounts of waste. 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 an academic aquarium at a university, using a variety of tank sizes and styles to care for different fish. 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. for planted freshwater aquariums, it is also important to maintain a balance between the duration and quality of light, the amount of plants, co2 levels and nutrients. the amount of fish on the tank can also affect the nutrients levels. for a given amount of light, if there is insufficient number of plants or insufficient co2 to support the growth of those plants, so as to consume all the nutrients in the tank, the result would be algae growth. while there are fishes and invertebrates that could be introduced in the tank to clean up this algae, the ideal solution would be to find the optimal balance between the above-mentioned factors. supplemental co2 can be provided, whose quantity has to be carefully regulated, as too much co2 may harm the fishes. aquarium classifications an aquascaped 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 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 includes fish from temperate areas worldwide. a saltwater aquarium aquaria may be grouped by their species selection. in a community tank, 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 and each other. aggressive tanks, by contrast, house a limited number of species that can be aggressive toward other fish, or are able to withstand aggression well. most aquarists maintaining marine tanks