Salt Mixes

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THINGS YOU DON'T KNOW ABOUT SALT MIXES
All salt mixes are not alike.  They vary in quality of starting materials, percentages of components and additional additives not normally found in sea water.
STARTING MATERIALS:
The major component of salt mixes is sodium chloride or better known as table salt.  It is mined in enormous quantities and purified in different ways.  For aquarium salt, they use a "technical grade".  An old joke said "Technically it's salt" and it is 99.+ % pure.  The other percent is comprised of other minerals and things like dust and clay.  When you mix up a batch for your tank you often find sediment in the bottom of the mixing tank if you turn the pump off.  This is harmless like dust on the furniture.  There are other chemical grades listed below FYI.  Considering the composition of natural seawater, technical grade is good enough and it is cost effective.  If a salt manufacture says they use more pure chemicals than the others, it's only a fraction of a percent more pure and has little or no effect on the final product.  Other components are usually no more pure than lab grade. 
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QUALITY CONTROL:
This is one aspect few think about.  There are specific percentages of each part of a saltwater mix.  Let's take iodide for example.  Natural seawater has 0.06 % iodide content.  This amounts to 1.6 grams of sodium iodide added to a 1000 pounds of sodium chloride.  If you want to cut costs or you don't care about consistency, a few tenth's of a gram makes a significant difference and iodide is much more important than calcium.
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DO YOU REALLY KNOW WHAT'S IN YOUR SALT MIX?
Let's pretend you mix 100 pounds of sodium chloride (table salt), 5 pounds of calcium chloride (Turbo Calcium available at Home Depot in 50 lb bags under $10)) and 25 pounds of magnesium sulfate (Epsom Salts).  This is about enough mix for 450 gallons.  When you test the water, calcium will be 400+, pH will be 8.3, and magnesium will be really high.  Everything you testing maniacs want will be there but it will be the worst mix you could get.  My point is:  TEST KITS REALLY DON'T TELL THE WHOLE TRUTH.
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THINGS THEY DON'T TELL YOU:
You don't need dechlorinators when using salt mixes.  (You shouldn't use tap water anyway and RO and/or DI water has already removed any chlorine.)  A very small amount of sodium thiosulfate is in the mix which destroys any chlorine.  Second, salt water easily reduces things like chlorine to harmless chloride.
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FAST DISSOLVING SALT MIXES:
This hobby is not for those who lack patience.  When mixing a new batch of saltwater, it should circulate for 24 hours.  During this time all the components are able to dissolve completely.  This also lets the 
pH and alkalinity stabilize.  Brands like Instant Ocean, Reef Crystals, and Oceanic (and others, I have been informed) use an additive called EDTA (EthyleneDiamineTetraacetic Acid).  To keep it simple, things dissolve faster with it.  It is also a cumulative poison that is absorbed by corals.  Eventually they are unable to take up food.  We sell Crystal Sea that does not include EDTA which leads us to the next problem. 
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CORAL BLEACHING
Common reasons for coral bleaching include:
Not mixing salt long enough - 24 hours is necessary for stability.  Less mixing may leave solids that can settle on corals and burn their tissues.
Changing salt mixes too quickly - water changes should be 5 to 10% initially.  If your water has lower than normal pH and it jumps quickly it hurts corals.  Large water changes remove organics from the water that act as light filters (a green color to the water).  The intensity of light increases noticeable with a large water change.

In order of importance, these things affect inhabitants in your tank from most critical to least critical.
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ammonia greater than minimum on most test kits
nitrites greater than zero
pH under 8.2 or above 8.6 (average reading between morning and night)
Nitrates greater than 40 ppm (LESS CRITICAL IN FISH ONLY TANKS)
temperature sustained under 68 or sustained above 80 (see page on heaters)
salinity too high (1.026 or greater)
alkalinity lower than 240 ppm (4.8 Meq/L)
light being too low or too high (coral/reef tanks only)
salinity too low (less than 1.020 for reefs, less than 1.016 sustained for fish)
voltage in your tank above 2 volts AC (induced, shorts in equipment, not having a grounding wire)
circulation

CHEMICAL GRADES LISTED FROM HIGHEST PURITY TO LOWEST.

ACS
Highest quality; often equals or exceeds the latest purity standards set by the American Chemical Society (ACS). This is the only universally accepted standard. Chemicals are of the highest purity attainable.
  
Reagent
Purity is generally equal to ACS grade. This grade is suitable for analytical work and is more than adequate for general lab use.

USP
A grade of sufficient purity to pass certain tests prescribed by the US Pharmacopoeia (USP); acceptable for drug use. USP grade may be used for most laboratory purposes.

NF
A grade of sufficient purity to meet the standards of the National Formulary (NF).

Lab
An upper-level intermediate quality. Exact impurities may not be known; however, lab grade is usually pure enough for most educational laboratories.

Pure
Purified, pure, or practical grade; a lower-level intermediate quality. Although this grade does contain impurities, it is usually pure enough for use in educational laboratories.
  
Technical
A good-quality grade used industrially. Use caution when substituting for reagent-grade or lab-grade chemicals.