Well consider what you are keeping. If you are going sps or something that needs a lot of flow, you are better off with a coarse type of sand like the special grade reef sand. If you have lower flow, than something like bahamas oolite, which is super fine white sand, looks great, but it does tend to get moved around by water flow pretty easy. The clam won't really care what type of sand you have and depending the the species, he might prefer to be in the rocks anyways. Good luck