No need to add anything below the sand, and heavy root plants will do fine in sand. Pool filter sand(PFS) is what you should use, not play sand. I buy mine in 50# bags, which I realize is overkill for you, but it's cheap and you may pay a premium for smaller amounts. Lots of people use the HTH PFS from ACE Hardware, 12.99 for a 50# bag. I use AquaQuartz personally, pay a little more for that.
Last edited by mgeorges; 02-27-2018 at 02:44 PM.