"There are no written rules of how to keep corals" There's actually a lot of info out there including Borneman's tome on corals where he stresses the importance of good husbandry and stability in water quality that keeps corals like Acropora healthy. Your tank looks good, but there's something going on that is causing your coral to lose tissue. Many aquarium and reef retailers will test your water for free. Doing some baseline tests for several days will reveal how much alkalinity, calcium, and magnesium are being used by the corals over time. Then you can figure out how much of the appropriate additive to add. My best guess is that your tank experienced a spike in alkalinity. Is your tank low in nutrients? If so, slight changes in alkalinity and or calcium can cause the symptoms that you observed with one of your corals. Remember--stability and water quality are essential for success with SPS.....................Jim