For the most part underground fiberglass tank manufacturers have similar backfill requirements. The standard backfill is pea gravel or crushed rock. If using pea gravel it must be 3/4 minus. Three quarter minus is round, washed, free flowing rock 1/8”-3/4” in diameter. No more than 5% may pass through a #8 sieve. If you decide to go with crushed rock the installation guidelines is going to require 1/8” - 1/2” diameter.
Sometimes the construction zone has ideal native soil, and in efforts of saving money the contractor may elect to use it. The contractor would send in a sieve analysis to the manufacturer to get a alternate materials approval. You can acces the PDF download version of our backfill guidelines in our library.