I have some large chunks of marble in one of my plant tanks and the plants do very well. Shell is calcium carbonate as is marble and it is not very soluble and will realy only go into solution in acid water. Some plants like that and some do not so it would be a matter of try and see. I have some Echinodorus sp. as well as Hygrophila, Rotala and Crypts that tolerate hard water like balansae,wendtii,walkeri and petchi.