Ive used play sand in a shrimp nano and it was inert silica sand as far as I can tell. Ive never heard of anyone having issues with it other than anaerobic patches if its not maintained properly. The most popular ones are B&Q, and Argos own brands but I used early learning center stuff. The great thing about play sand over pool filter or blasting sand is that the grains have to be rounded so it feels quite soft, its meant to cause less damage if it gets in a child's eyes.