Reef Crystals are mined from dry ground....with the salt comes gypsum. That is what the grey deposit is in you mixing barrel, power head, etc.
Red Sea Salt is made directly from evaporated salt water from the Red Sea.
~95% of salt mixes contain sodium chloride....the calcium, mag, alk, and trace minerals make up only a tiny amount of the whole.
There is another brand name that is made from the Red Sea salt as well....Royal something.....I'll have to look back.I've got a thread about it. But their American rep gave me a good idea on how these salts are made, and how difficult it is to get consistant coverage by the additives like above to make your first cup full the same as your last.
So even though it can be a bargain to buy larger pails/bags, the smaller units will be more consistant overall, because they tend to be used totally in mixes.