Argos play sand was fine for us. Did need to wash it though, more than I expected - it looked clean, but clouded up when you add water. After lots of rinsing in a bucket, the cloudiness disappeared. After adding to the tank, a blob of filter wool in the filter took care of the last traces. I'm so glad I took the trouble to wash it properly, as the water in the tank has been crystal clear since then, even after stirring up the substrate while cleaning.