Most likely a water problem.
Can you answer some questions for us?
A. Total water volume ( Please include sump, etc.)
B. What are your parameters? ( CA, ALK, PO4,Salinity, etc.)
C. Type of lighting. Please include age, watts, and bulb/ballast name.
D. Filtration method (Skimmer, GFO, DSB, etc.)
E. What types of corals do you keep?
F. How long has the tank been running?
G. What are you dosing? Dosing schedule?
H. Which brand of salt are you using?
I. What brand of test kits are you employing?