You can also use those 'vacation fish feeder' blocks, as they are mostly plaster, which is calcium, and they'll eat those, and build up their shells. It is pretty common for the shells to get some deterioration as they age, but so long as the pH is above 7, they should be ok, if the water is reasonably hard.
Soft water is a problem because there's so little calcium in it. And I put cuttle bone in too. It sinks after awhile and eventually disappears, whether they eat it or it dissolves I am not sure, but you don't have to put it in the filter. It will catch a lot of crud in there, due to the texture of the cuttle bone.