I have been "told" before that different salts contain different qualities of minerals, particularly in calcium. The idea is that some salts that may read high in calcium aren't necessarily efficient as it is possible to get a false indication of how readily absorbed the calcium in that particular brand is. Without naming a specific brand, I had once used a salt that seemed to maintain my calcium much more easily than others. If it is in fact possible to have different qualities of salt, theoretically, and according what I was told, the calcium measurements I was getting and their stability was due to the fact that the corals were not uptaking what was in the water column.
Can anyone confirm this theory with any hard evidence?
Not sure if you are still looking for an answer, but that isn't true. There are no current mixes that I am aware of that use calcium chelated with organics, which would be stupid and very expensive. That's the only way it could become less "available".