What type and size of sand should I use in the Bestway Flowclear 58515?

You must use #20 silica sand sized 0.45–0.85mm, free of all limestone or clay, and approximately one 8.5kg bag should suffice. Using the wrong size sand will reduce filtering performance and may damage the sand filter, voiding the warranty. As an alternative, you can use Flowclear™ Polysphere, using 28g to replace every 1kg of sand. (Page 5)