<P> 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). </P> <P> 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). </P> <P> 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 . </P> <P> 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 . </P>

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