substrate

[Tarolisol]

Im not sure if it offers the same kind of biological filtration as sand as it is a bit larger you will have to wait for more people to chime in here, crushed coral comes in many sizes from very large to a little bigger the sand. In mine i see little creatures living in it so maybe it does do the same stuff as regular sand, but dont quote me on that. I know it takes alot for my power heads to blow that stuff around.