The batch of SD play sand that I had the misfortune to put in my tank was contaminated with some substance which turned to cement in my tank. Within hours after adding the play sand, a thick cement layer formed on every internal surface of the tank, filter, sump, overflow, probes, etc. It had to be chiseled off and made a huge mess. This same thing happened to at least one other person I know of. Based on my personal experience with this stuff, I do not believe SD's warning is purely a marketing ploy, as some have hypothesized.