Where can I find the installation instructions for my below ground tank? Posted on April 21, 2016 by Mark Peabody The installation instructions can be found under the edge of the lid. Mark Peabody Is there a direct correlation between the number of bedrooms and the size of the septic tank? You refer to a sand/gravel mixture in your installation instructions. Exactly what type of sand/gravel mixture do you recommend for bedding/backfill?