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Aquarium water chemistry is of vital importance to the health of fishes and other aquatic species. Aquariums may have freshwater or saltwater.

Saltwater, either natural or artificial seawater, is used in fish-only, fish-only with live rock (FOWLR) and reef tanks while freshwater is used in planted, biotope, cichlid, brackish and predator tanks. 

In general, the former is more expensive and more difficult to maintain as additional equipment and frequent testing is needed. To ensure good water quality, maintenance (filtration, water changes, and testing) is required on a regular basis.