I have a 20g with Mystery and Nerites. The 2 big Mysteries have some damage on the open end of their shells, but they appear to be repairing themselves with something of a different color than the rest of the shell. The one medium sized ivory Mystery has a clean ivory color on most of the shell but the newest portion is dirty colored with a seam down the middle. I've been adding cuddle bone since the big ones started showing damage 3 weeks ago. Ivory has been in the tank about 2 weeks. I have 1/2 of a cuddle bone in the tank, but they don't eat it. Is 1/2 enough? I've read that they will eat flavored Tums. Do they need to eat it? Nerites seem fine.