Back in the Stone Ages, when I worked at a LFS, we had a guy in our warehouse that made salt. Back then, the only competition was IO. He was a chemistry professor, and he'd gotten his PhD on the analysis of seawater. Surprisingly, it does vary throughout the world. He also wrote THE column on SWF for Tropical Fish Hobbyist - which was the magazine of the hobby back then.

He actually added gold to his salt mix - just to say it was there. We bought it from him in bulk, and repackaged it.