I had a similar episode last summer. Lost 80% of my SPS over 5 days and I hadn't even changed anything for months. My conclusion is the same as untamed and aquottro's. Some infection or illness perhaps. Remember that these are close nit colonies. All it takes is one Polyp to catch a cold and the colony is hooped :-(. You can actually see it as it unfolds. It usually starts with a very small area that gets bigger and bigger from that point as it spreads like wildfire. If you nip off the tips or branches before the infection has reached it you can sometimes save it.
Anyway, I never changed anything during and after that even. Tank still has the same flow, biopellets, carbon, gfo regime, same old same old. Although over the past few months I have been using a new salt with every water change. I rotate through Reef Crystals, H2Ocean, Royal Nature and Aquavitro. This is not for the faint of heart :-). Everything seems to be okay though.
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