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As the saying goes, "It's the water" and a whole lot more. Seawater is a complex mix of living (microscopic and our livestock) and non-living (water, salts, organic and inorganic "trace" materials...) matter. Some substances are remarkably constant in natural seawater, yielding a consistent composition worldwide. The same cannot be said for small, captive systems; some aspects of their make-up change slowly (pH, alkalinity), others are more dynamic (redox). What's more many physical, biological and chemical constituents change in concert/opposed to one another. One famous and important example of this is the relationship between ammonia and pH. The toxicity of this aquatic waste product is elevated orders of magnitude by higher pH.
"Nature abhors a vacuum" certainly applies to marine water. It is filled with detailed interactions. The oceans are vast, dynamic, mixed continuously and constant in composition. Our system's cannot, do not, and don't have to do more than approximate what natural seawater is. Synthetic mixes can provide all aspects of a supportive medium; most systems without the addition of any further supplementation.
Adjuncts: To add or not to add, that's a question
There is continuing speculation on the roles of inorganic ions of a number of elements and compounds play in closed systems. Some considered necessary for life are called essential trace materials; trace because they are found in such slight concentrations.
Two choices exist for sourcing the elusive medium, seawater; natural and synthetic. The basic pro's and con's of both have been covered in D) 1). Real saltwater is used by very few aquarists due to it's many serious drawbacks; costs for hauling, treating; lack of life-support over a relatively short interval; dangers of introducing undesireable pests and pollution... There are few exceptions, like the Bishop Aquarium in Hawaii, and other coastal large-volume open-system users, where natural water is a viable alternative.
For small, closed systems, artificial seawater is highly preferred; it's convenient, cost-effective, and supports all forms of marine life.
About Artificial Seawater and Supplements:
Some of the original questions facing investigators/formulators of synthetic seawater was what are the roles of such materials in captive systems? How necessary is trying to match the sea's make-up of the same? Do they build-up (with addition of food, other sources), decrease in concentration (due to bio-accumulation, selective up-take, precipitation...) over time?
The first synthetic formulations met with little success biologically. These mixes were chemists initial attempts at exact duplication of nature, and they didn't work.
Starting in the 1950's in Germany, moderately successful mixes focused on imitating only the oceans major constituents; adding small amounts of "trace" materials only as they were apparently needed.
In the U.S. (1961, 62) W.E. Kelley and Richard Segedi at the Cleveland Aquarium instrumentally improved on the German mix by lowering cost and further defining the concentration of certain trace elements. This is today the basic formulation used by all major/large distribution salt mixes.
Ocean seawater has all naturally occurring elements present in it. Good synthetics list @ seventy trace elements in various formats in their make-up; some increasing number of which have been deemed "essential" for life. It is a safe bet that more material will be considered necessary as time goes by. Happily, the process of making synthetics almost always involves utilizing "dehydrated" natural water that preserves some trace element material. Additional, known essential trace substances are intentionally added to the rough raw mix.
Viable synthetic sea salts do not, should not attempt to absolutely replicate seawater. The natural marine environment is a far more complex and uncontrollable medium than our closed systems can manage.
Artificial Media, Components, PR & You:
Some companies have gone to extraordinary lengths to produce pseudo-scientific (small, opened samples from the payee's source, no double-blind labeling/testing...) "reports" declaring their products superiority over all others. Don't be fooled by such claims of ocean-like water quality.
Whether a brand has two components, wet or dry, or even a slurry, does not make any appreciable difference in it's suitability as an artificial medium.
A good mix may contain added doses of trace elements, vitamins, calcium, even hormones; or not. It is my opinion and the bulk of the non-biased scientific literature's that unless you're engaging a crowded, "reef" environment with boosted light and filtration or intensive closed-system aquaculture facility, the addition of supplemental "trace" materials is not warranted. For differing opinions (there are many) you are referred to any/all of the "reef" books and column writers in the pet-fish literature.
Sources of Trace Materials:
First from the express formulation of the mix, and intentional and unintentional contamination of their constituents. To a minor, but possibly substantial account, from the source water (tap) used to make up the synthetic.
Important minerals such as calcium and silica are frequently available from substrate, coral, rock and shell decor. Calcium is often super saturated (400-500 plus ppm) in an adequately set-up and maintained system. Adding more will not, does not increase it's concentration. More about this element in the next section.
Vitamins are present in sufficient quantities and qualities in properly prepared and stored foods.
Hormones are produced, secreted and excreted by the life in the system itself. They do not need to be added externally.
Sources of Loss of Trace Materials:
There are records in the literature that detail the loss of trace elements in the absence of purposeful livestock. Interactions with glass, plastic aquarium walls, substrates, and more are responsible for iron, calcium, strontium... leaving solution.
Dangers of Supplementation:
There are several possible dire consequences of purposely "enriching" your water. Some nutrients can be toxic by themselves; others act synergistically or antagonistically with others, some cause co-precipitation.
Thankfully, most commercial supplement products are so dilute as to be considered placebos. Pouring them in does little harm, and tantamount good. Be warned; more systems by far suffer from accumulation of "trace" materials than a lack of them.
An example that will be mentioned is the trace ion nitrate (NO3) for raising and rearing young fishes and invertebrates. Reducing nitrate by dilution, under-crowding, under-feeding are requisite for breeding and raising marine livestock.
What You Can/Should Do:
As stated above, unless you're "pushing" a reef set-up, don't sweat your system suffering from a deficiency from not periodically pouring in ACME A,B,C supplements. The best way to avoid composition imbalance in a closed system is still, under-crowding/feeding, proper filtration, and frequent partial water changes (10-20-30% or so regularly) every few weeks depending on system size, loading, filtration... This practice safely replaces calcium and trace elements used by your livestock and beneficial microbes, while reducing wastes and metabolites. You do add all other necessary trace elements; by feeding.
Yes, there are elaborate filtration and supplementation schemes that will allow you to stretch water quality, but there is nothing that compares with the safe, effective, cost-saving practices of the above principles.
Please don't be bothered to the point of not trying, or giving up on the marine hobby as a consequence to all that is spouted about the relationships between micro- and macrobiota, and water chemistry. There is more than a lifetime currently known on these interactions, but you don't need to be aware of them to have fun keeping a captive system going.
The next two installments offer a survey of the most commonly discussed major and minor "trace" materials.
Atz, James. 1971. Some principles and practices of water management for marine aquariums. Marine Aquarist 2(1):71.
Ederle, henry. 1992. Modern Sea Salts. T.F.H. 10/92.
Segedi, Richard M. 1975. Trace Elements in closed systems. Marine Aquarist 6(6):75.