I want snails and have added several to my tank of guppies,. etc...They are not doing well. Their shells are flaking and getting holes. I researched enough to know the calicum level needed to be higher and put a cuttle bone in the bottom of my tank held down by tank decorations, but the snails after 4 days, have not paid any attention to this cuttle bone. What should I be doing to help these snails, but not interfere with my fish?