Missed the fact that you were in Chile... Hey, if you can't get it any other way, and it is legal, then yeah, I'd collect it. I'd probably try and find a clean patch, underwater, with healthy critters living in it, and put it straight in the tank.
http://www.washingtontimes.com/news/2013/aug/26/florida-high-alert-beaches-run-out-sand/
http://abc7.com/news/residents-to-pay-$31m-for-sand-replenishment-at-malibus-broad-beach/1052962/
http://www.nytimes.com/2007/09/16/travel/16heads.html
My google-fu is strong

It's a common problem. Storms regularly wash beach sand out to sea, and communities count on nice, sandy beaches to attract the tourists.