You refer to a sand/gravel mixture in your installation instructions. Exactly what type of sand/gravel mixture do you recommend for bedding/backfill? Posted on April 21, 2016 by Mark Peabody The sand/gravel mixture we refer to can generically be called a “pea gravel” mixture. It should be 100% smaller than 1-1/2″ and approximately 50% smaller than 1/4″. Mark Peabody Where can I find the installation instructions for my below ground tank?