Some ceramic items have heavy metals, especially lead, in the clay or the glaze, may be what is killing the corals. See below -
While the majority of lead poisoning is suffered at the workplace, the Centers for Disease Control and Prevention (CDC) has warned that dangerous lead exposure can also come from some very unlikely sources. Things like tableware and traditional Indian medicines, as well as other unsuspecting sources have been cause for concern. The agency's Morbidity and Mortality Weekly Report journal outlined two unusual cases of lead exposure.
In one instance, 12 people became ill on separate occasions after using ayurvedic medicine, which are traditional healing products derived from India. The majority of the victims ended up in the emergency room just days after using the medicine with vomiting and severe abdominal pain. The lead found in their blood was far above what the CDC considers safe. One woman in particular was taking an ayurvedic pill for the purposes of increasing fertility when she had a miscarriage and an extremely high level of lead was found in her blood.
The other CDC report was about a boy that went in for a routine examination that uncovered high levels of lead in his blood. When the source of lead was not found anywhere in his house, the French ceramic dishes he was using to eat were examined. Defective glazing or firing allowed lead to leak from the dishes, which is a more common occurrence in tableware made from Mexico or Latin America.