Check out this site:

http://www.rshimek.com/reef/sediment.htm

It's and article by Ronald L. Shimek, PhD. Apparenly he knows EVERYTHING about marine invertibrates. I think a reference book that he wrote just came out recently.

Anyway, here's my synopsis of the article...

The only thing that really matters, when it comes to the sand, is the particle size. Most play sands will have a particle size that is pretty close to that found in nature. But, most play sands are silica based. (Whether silicate leaching happens or not is still up for debate, and you can read all about that if you keep looking under "deep sand beds" in a search engine.) The only real problem with silica sand is that the particles DO have some sharp edges that can damage the sand inhabitants. He says it's not a big deal, though.

Of course, the best sand to use would be an aragonite sand. This is due to its PH buffering capabilities. Here's where the whole Southdown/YardRight/OldCastle deal comes into play. You can read all about that, too, in a search for "Play sand." The way I understand it is that OldCastle bought out Southdown, who also packaged under the name YardRight. The aragonite sand is called "Caribean Play Sand" and it's mined down there and then shipped on barges to PA and FL. So, it's hard to find it here in TX or really anywhere but the east coast. Since the buy out, if you by OldCastle play sand, you really don't know if you're getting the aragonite sand, or if its silica sand.