from what I have read it is fine, if you look at my old post (Quickcrete playsand fine for tank?) you should be able to find all the info you need. Also read this article on the subject: <a href="http://www.reefs.org/library/article/r_toonen9.html" target="_blank">http://www.reefs.org/library/article/r_toonen9.html</a>