Hi and welcome to TONMO.com
Bimacs live off the coast of Californa (from mid-coast down through Baja California in Mixico. They live in mud and sand and sometimes on rocky reefs.
Most people keep them on a shallow sand bed, being careful not to use sand that has sharp edges. I researched this a couple of months ago and posted in this forum. This is what I wrote:
I spent some time on the phone with the Carib Sea people about this question(which substrate).
They make a very fine sand Aragamax that's almost like mud. There is one with slightly larger grain size caled Aragamax select. Both are considered "sugar sized" with a grain diameter of 0.2 to 1.2 mm. Neither would have sharp edges and are recommended for sharks and rays. Another slighly larger size grain in offered in the Special Grade Reef sand (1.25 -1.95mm )
The live sand costs more, but even if you don't need live sand, it can be an advantage because you don't have to wash it for an hour. They offer Bahamas Oolite, Fiji Pink, and Special Grade Reef sand - these don't have sharp edges.
Colin has a good suggestion about mixing the grades. I'm going to mix Fiji Pink with the Special Grade Reef Sand.
Nancy