Read this on a forum post. As many aquarium salts are mined, it wouldn't be surprising that a small amount of clay could get mixed in.
i've recently switched from Red Sea Coral Pro to Reef Crystals. reef crystals does disolve much faster and the price is lower, the thing that strikes me is what appears to be contaminants in the salt! i mix my water in a snow white bin and would wash out with fresh RO
every three or four weeks with the coral pro salt but have to wash it EVERY time with reef crystals, it leaves what looks like dirt all in the bin. that spooks me to say the least but as i got a freaking boatload of it at once i'm kinda stuck with it... are you doing anything to clean this up?
and their responce...
It is common to find small bits of debris. It is completely harmless and nothing to worry about. Since salt is mined, it is nearly impossible every little particle of debris.
United Pet Group, Aquatics Div.
Senior Consumer Relations Specialist