1. Temp of tank?
2. All parameters including , Alk, PH, Mag etc and even Calcium in my opinion
3. Salinity?
4. Is it a frag, colony, new addition, wild caught or not, shipped or pick up?
5. Have you made any changes just prior to or during the issue?
6. List the current/flow, even lighting when applicable?
7. Additives?