sand. though they are fairly hard to keep. there is a reason why they are not very common in the hobby. they can die and foul up the water and you wont even know about it.
they are also filter feeders.. without waste and nitrites and other nasty goodies to eat.. your tank will be shortly lived.. that.. and when they die.. nitrate spike from ****... id seriously reconsider something else if you can..
before we could tell you what else would be good in your tank, we need to know the size of your tank and how long your tank has been setup and if you already have any other fish in there and what they are. Also what kind of setup you are going for.