Welcome to TONMO, the premier cephalopod interest community, and birthplace of #WorldOctopusDay and #CephalopodAwarenessDays. Founded in 2000, we are a large community of experts, hobbyists and enthusiasts, some of whom come together when we host our biennial conference. To join in on the fun, sign up. You can also become a Supporter for just $50/year to remove all ads and enjoy other perks. Follow us on Twitter for more cephy goodness.
The type of sand depends on the species. Generally you want sand without sharp edges. This is what I wrote some time ago about researching substrate for a bimac (about 1 inch deep):
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 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.