Water, especially seawater, is a highly complex liquid. Although sodium chloride (table salt) is the most abundant compound in natural seawater, trace elements, organic and inorganic compounds, and other chemicals are all present in varying quantities. Seawater and freshwater have calcium carbonate, magnesium sulfate, and elements such as magnesium and zinc. Organic compounds and microorganisms such as plankton give water from different areas unique characteristics.
Are there certain aquatic creatures that must be kept in natural seawater or freshwater?
Delicate creatures from very isolated habitats (such as caves) where the water has unique chemical characteristics generally do best in water taken from their natural environment. Natural water is useful for raising tiny fry or filter-feeding invertebrates, because it contains the tiny organisms upon which these animals feed.
Although major public aquariums located near the seashore often use natural seawater for their exhibits, this is generally a difficult prospect for the home aquarist. The planktonic organisms found in both freshwater and seawater generally perish in captivity, and their deaths cause harmful chemical changes and an increase in ammonia levels. It is also possible to introduce pathogens, parasites, and disease to your aquarium through the use of natural water. Pollution is, unfortunately, also an overriding concern.
If you do choose to use natural water, collect it from as pristine an area as possible. Generally, seawater collected offshore or from ocean beaches will be safer than that from enclosed areas such as bays and lagoons. The same concept applies to freshwater. Be sure to store the water in containers that have not contained harmful chemicals, which may remain in plastic even after a thorough washing (spring water bottles are a good choice). The safest method of readying the water for the aquarium is to leave it, unfiltered, in a dark area for a week or so, so that the resident microorganisms can die off. Chemical tests should then be performed to determine ammonia levels and pH, and these should be adjusted as necessary. The water should be filtered for at least twenty-four hours before use.