I tried the cuttle bone as well since many people suggested it to me. I boiled them for a long time (I got the huge cuttle bones) and they did sink after a good boiling. My snails have been nibbling on it on and off today 
Crushed coral or agonite (sp?) sand or crushed coral will work with raising your Kh and Gh levels so I would go out and get one or the other. The calcium foods will also help. I am currently adding liquid calcium, cuttle bone, agonite (sp?) sand to the filter, and high calcium foods since my snails clearly need a bit of calcium to get their shells in good order
Crushed coral or agonite (sp?) sand or crushed coral will work with raising your Kh and Gh levels so I would go out and get one or the other. The calcium foods will also help. I am currently adding liquid calcium, cuttle bone, agonite (sp?) sand to the filter, and high calcium foods since my snails clearly need a bit of calcium to get their shells in good order 