Originally posted by OrionGirl
Beach sand will probably contain a lot of calcium, and will likely raise the hardness of the tank. Also, you're going to have to rinse much more than you think to remove all the salt. I wouldn't advise this for another reason--the beach sand is going to bring with it a wide variety of contaminants (ie, oils and such) in addition to possibly having dead critters, which will pollute your tank.
When sand is gathered for use in SW systems, it's usually done from 1 mile from the coastline, to avoid the pollution concentrations that wind up in the shallower waters.
