This morning I noticed 1 of my 3 mystery's had about a quarter of its shell missing by the foot. I checked the other 2 and the baby snail's shell was also brittle by the foot but the other appears fine. I checked my kh/gh and the kh has decreased some from the last test. Last test the kh was 10 drops, today it was 7. Gh is the same-11 drops. They get sinking shrimp pellets algae pellets, and cucumber plus any fish food that my pesky tetras don't eat. Any suggestions?