Have been doing a little research on salt this morning, not related to this thread. I began at this maunfacturer's website. Start reading down the right hand side starting with the article "How to Judge a Good Marine Salt".....
http://www.aquacraft.net/
Then, for a review of the website.....
http://www.calgaryaquariumsociety.co..._Aquarist.html
And a few words from WetWebMedia.....
http://www.wetwebmedia.com/seah2ofaq4.htm
FWIW, for my money, I'm not interested in any of the higher prices salts. I don't believe they are worth the extra money. I'd just as soon buy IO and be done with it. Alk is easy to adjust with a combination of inexpensive baking soda and "baked" baking soda. And for Mg and Ca, I can get these additives from littlesilvermax (ChemMaster) here on this board.