Fish and other marine organism are directly influence by the chemical, biological, and physical characteristic of their environment. The water the constant current, wind and other factors that maintain relative uniformity of the water mass. As a result, reef organism are not subjected to wide fluctuations in the chemical and water. When there is a change itis usually of short enough duration that the animals are not adversely affected.
The situation in an aquarium is radically different. The marine aquarium water is not subject to constant renewal as in the ocean. In contrast to a coral reef, aquarium water is subject to extensive alternation after the introduction of marine organism.
The alternation of marine aquarium water is due to the build up of chemicals that originate from various biochemical processes, most importantly from the metabolic activities of fish, invertebrates, and algae. If these chemicals are permitted to accumulate to concentration beyond what the marine animals can tolerate, the survival of the inhabitants will be placed in jeopardy. The toxicity of some of these chemicals is lethal to the inhabitants in very low concentrations. So regular testing of marine aquarium water and periodic water changes are crucial for maintaining water quality within acceptable parameters.
Seawater is an extremely complex solution composed of numerous chemical compounds, both organic and inorganic. Sodium chloride, or common table salt, is the most abundant inorganic compound in seawater, followed by other major component such as magnesium chloride, magnesium sulfate, and calcium carbonate. Smaller concentrations of trace elements are also dissolved in seawater and include molybdenum, selenium, cesium, vanadium, and zinc . though these elements are in extremely low concentrations, they are important to biological processes of many organism, including algae and invertebrates.
Marine aquarium can be filled with either natural seawater or freshwater plus a synthetic seawater mix. Natural seawater must go through a conditioning process before being used in aquarium. Natural seawater contains numerous microscopic organism, both plant (phytoplankton) and animal ( zooplankton). If the water is not properly conditioned by dark storage for a few weeks, the death of these organisms could radically alter the chemistry of the water, endangering you aquarium specimens. Secondly, unconditioned water could transmit infectious diseases to your marine fish. Some aquarist prefer to collect their own water, but it is safer and more convenient to use a good quality synthetic sea salt mix. This is especially true for the new marine aquarist who will find the storage and filtering procedures bothersome and time consuming.
If you decide to collect your own seawater, it must be collected away from inshore areas that could be polluted from fertilizer runoff, sewage heavy metals, insecticides, or other pollutants. Inshore water also often contain large quantities of suspended particulates. Collect seawater only i n nontoxic plastic containers with good fitting caps. The water should be stored in closed plastic or glass containers for several weeks before use. After storage, you will note on the container bottoms a fine layer of sediment that should not be added to your aquarium. After addition to the aquarium, the water should be subjected to filtration and aeration for several hours before addition a fish. It also recommended that several water tests be conducted. Including pH, ammonia, nitrite, and nitrate to insure that they are within safe levels.
The invention of a synthetic sea salt mix for keeping marine fish and invertebrates in aquariums was a major achievement that enabled anyone anywhere to set up a marine aquarium. It allowed the convenience of preparing seawater by mixing freshwater with a salt mix. The earlier formulations were inadequate to support fish, and especially invertebrates. Some formulations used inferior grades of chemicals and often some salt didn’t dissolve completely. Today there are various excellent commercially produced salt mixes that are readily available in aquarium and pet shops. Experience has demonstrated that these synthetic mixes are superior for supporting marine life.
The major of advantages of preparing synthetic seawater are that it is free if pollutants and micro organisms that could transmit disease or foul the water, there is no need for storage of extra seawater, and the preparation can be used within a short time after mixing.
source : The Saltwater Aquarium Handbook, by : Blasiola, George C.