I believe that most fish that nudge in the substrate like sand as long as it isn't sharp. You want to avoid any substrate that can damage their barbels.
I have sand in all my tanks. My clowns especially love it. When I had gravel in their tank, I would notice small white patches on the fronts of their noses...from digging through the rocks, so I changed to sand. Personally I think it looks better and is more natural for most fish. Opinions vary though :