Here is some info I found on the Seachem website, it's a 3meg document so I copied and pasted the text -
Seachem Laboratories: Innovative Pet and Aquarium Products
THERE ARE NUMEROUS SALTWATER MIXES AVAILABLE TO MARINE AND REEF
hobbyists. The marketing strategies for these salts sometimes makes
an informed and rational judgment difficult. What characteristics should
the ideal salt have? Authentic sea water is a good starting point for
judging any salt, although rational considerations for deviating from
natural seawater should not be dismissed, since a closed aquarium is not
the open ocean or even a reef. Some components in natural seawater may
be of dubious benefit or even be harmful in a closed environment.
What characteristics of the ionic composition of natural seawater
complicate duplicating its composition as a solution or a stable dry
product? The key word that describes saltwater is ionic. This means that
dissolved components exist in water as hydrated or dissociated ions. When
you add sodium chloride to water you do not have a “sodium chloride”
solution, you have a solution of distinct hydrated sodium ions and distinct
hydrated chloride ions. If you now add some “magnesium sulfate”, you
have distinct sodium, magnesium, chloride, and sulfate ions. If you were
to dry this mixture, you would not recover the original sodium chloride
and magnesium sulfate, but a mixture of sodium chloride, magnesium
sulfate, sodium sulfate, and magnesium chloride. If you were to add
“calcium chloride” to the solution you would end up with a solution
containing distinct sodium, magnesium, calcium, chloride, and sulfate.
Depending on the ratio of chloride and sulfate ions, you could easily
obtain a precipitate of calcium sulfate, which does not readily dissociate
and is thus relatively insoluble. If you were to add some carbonate, you
could easily get more precipitate as non-dissociable calcium
carbonate and magnesium carbonate. Likewise, if you
were to take natural seawater and dry it, you
would not end up with a mixture that, if
added to distilled water, would restore the
natural seawater. Drying seawater results in a
multitude of insoluble components.
So, duplicating seawater is not just a
matter of adding a fixed set of constituents. There
are several pathways available and each has its own set of
advantages and disadvantages. This is further complicated by the need
to have a solid mix rather than a solution. Many of the potential
component salts that can go into a salt mix are hygroscopic and many are
reactive. This complicates the problem because some of these can easily
interact in the solid mix and form insoluble components . One solution to
this is to add a coating agent such as fumed silica. Small amounts of this
extremely fine material soak up an incredible amount of moisture while
staying dry, and also coat and protect components in the salt from
interaction. Although, in theory, such silica is insoluble and should not
contribute silicate to the water, mounting anecdotal evidence suggests
that the material may not be totally insoluble in seawater. An alternate
approach is to decrease the amount of carbonates (alkalinity) of the salt.
This approach is very tempting, particularly with the current fad for high
calcium salts. With high calcium salts it is also tempting to lower the
sulfate content and increase the chloride content to minimize the
formation of insoluble calcium sulfate. Decreasing the magnesium
concentration also eases the task of preparing a salt with marketable
qualities. Selection of less troublesome components at the sacrifice of ionic
proportions that match natural seawater is an attractive option.
What, then, should be striven for in a synthetic salt mix? It should
contain all the major, minor, and trace components that are essential. That
is a given that can hardly be argued against. There may be some debate
about what is essential. Is borate, for example, essential? We think so, but
there are some who do not agree. All currently agree that phosphate and
nitrate should not be included in a salt mix, even though they are present
in natural seawater. Silicate is present in seawater at about 10 mg/L.
Should silicate be present in a salt mix, even though it may encourage
diatoms? A silent contaminant of commercial salt is ammonia, arising
from the use of magnesium chloride as a principal source of magnesium.
Many sources of calcium chloride are likewise contaminated with
ammonia. Consequently, most, if not all, brands of salt contain ammonia,
usually enough to yield between 0.1 – 0.8 mg/L in a freshly prepared batch
of saltwater. In most instances, this may not be a problem because the
ammonia is diluted by the existing tank water and the biological filter
should clear it in short order. But, it is definitely not a promotionable
feature of any salt, and, for that reason, has remained a well kept secret.
Must a salt be totally “dry”? That is desirable, but at the cost of
introducing silica or using components that cannot produce ionic
proportions typical of seawater? Must a salt yield complete dissolution and
instantaneous clarity? Again desirable, but at the cost of increasing
components that increase solubility and clarity and decreasing those that
compromise these features? Must a salt have a calcium content that
exceeds natural seawater by more than 25%? There is no scientific basis to
do so. Must a salt contain every trace element on the periodic chart?
Certainly not, and salts that recite an endless list of trace elements are
merely listing contaminants. Important trace elements such as iron,
manganese, copper, cobalt, and aluminum are fundamentally unstable in
seawater and need to be stabilized by complexing. Should a salt contain
vitamins? Except for promotional value, vitamins have no place
in a salt mix, if for no other reason that vitamins cannot
survive the harsh environment of a dry salt mix,
regardless of it composition, nor can they
survive long in the reconstituted “seawater.”
Salt mixes have recently multiplied almost
like rabbits. Many are just private label
versions of already existing mixes. Despite this,
Seachem has reluctantly introduced its own salt,
based on several years of evaluation of existing salts and the
properties of ionic interactions. Like all other Seachem products,
Reefsalt™ is produced in Seachem’s own facility under its own control,
using state of the art equipment and quality control procedures.
Seachem’s Reefsalt™ contains all the major, minor, and essential trace
components of natural seawater. It differs from most other salts in that it
does not follow the fad of elevated calcium. It delivers 400 mg/L calcium,
the natural seawater concentration. It delivers an alkalinity of 4 – 5 meq/L,
twice the natural seawater concentration, to provide adequate buffering in
a closed aquarium system. Since the salt contains no protective agents, nor
compromises on the proportions or type of component salts, it has a
slightly moist consistency. This is characteristic of some of the component
salts and is not a manufacturing deficiency. It is, however, loose, not hard,
and does not harden with age. The salt dissolves rapidly and virtually
completely, with excellent clarity, although it may require a few hours to
acquire full clarity. It contains no measurable nitrate or phosphate. Unlike
some other salts, it contains no measurable silicate or ammonia. The salt is
supplied in a sturdy and reusable hermetically sealed black pail, which
protects the salt from external contaminants and ionic interactions
catalyzed by light. Although it is not mass produced and does not
compromise on components, quality control, or even containers, Reefsalt™
is competitively priced with premium salts. Reefsalt™ lives up to the
reputation of its manufacturer, the first to bring quantitative testing for
iodide, strontium, and magnesium to the hobby.
