I have a tiny Mystery snail that has developed a small crack in it's shell above the head area. I was wondering if it will heal it self. The snail is eating fine and moves around the tank fine. I have no idea how it got the cracked as when I got it it was fine. Should I be concerned? And what can I do to help the heal the crack. I do put calcium tabs in the water. How often should I ad this to my water?