you will want to look for an aragonite based sand, not silica based. Aragonite is a calcium carbonate based. Silica based sand can contribute to diatom blooms, since they feed on silica. Not saying that silica sand is horrible for a tank but you may end up with more issues with diatoms than most hobbyists, if you use it. Typically, crushed coral is large chucks, similar to pea gravel (2mm+ granule). Crushed coral in this state is really too large for most tanks because it will trap detritus and cause a chemical buildup. It can be done but vacuuming on a regular basis is typically required to keep the nitrate levels down and to remove the junk that builds up in the rocks, similar to fresh water gravel.
