Thanks for your responses. I am fairly certain the probe is OK. First of all it was the condition of the corals that made me look for a cause and the only apparent problem was ORP which had dropped by 200 mV. The probe is floating in the return area of my sump/fuge and it was clean and unobstructed. Also a check a 400 mV cal. solution came out to very close to 400 mV. Now several days later, ORP seems to have stabilized at 290-325 but is not improving (nor getting worse). Corals are OK but my Xenia is less "plump" looking and my star polyp colony is only about 85% fully open. They all look good but not terrific as they did before. Still puzzled but there seems to be no immediate problem.
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