I'll answer for Tom...
Red Sea is the only salt that claims to be obtained from evaporated sea water. The others are all mixed from dry ingredients.
From the Red Sea web site:
<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote
Coming fresh from the Red Sea, the salt used to produce Coral Reef Red Sea Salt already contains trace elements in the same ratio that is found in natural sea water.
Sounds good! Unfortunately, it doesn't work.
When you evaporate sea water, the ions contained therein will combine into various compounds. They don't necassarily do this in the manner you want them to. For instance, there is Mg, Ca, and CO3 in sea water. You'd like the Mg to be bound to the carbonate, since that's relatively soluable at normal tank pH, making the buffer available. However, as you evaporate sea water there is nothing to prevent the CO3 from combining with the Ca, to make calcium carbonate which is
not soluable at normal pH.
In other words, if you take sea water, evaporate the water, and then add the water back, you don't get the same sea water you started with since some of the ions have formed insoluable compounds.
Furthermore, Red Sea's ads talk about it being more pure than other salts since it comes directly from the Red Sea. Last time I checked there was a pretty high level of petroleum waste in the Red Sea.
And lastly, we don't want to be bagging on them too badly, but...
Every single customer that comes into the store who has been using Red Sea test kits has water parameters messed up beyond words. If they can't make good test kits, I don't put a lot of faith in their salt.