Low oxygen. That would be a guess, except for the knowledge of power failures which make that guess an almost certainty.
When you get running, you have to simulate that power outage again. Watch what happens when you turn the power back on. Do ALL the pumps come back on? Does the skimmer come back on? Do you have a skimmer? If not, this episode suggests that you should because they will add O2 to the water, especially after a power outage.
The other thing you might need is a battery operated pump that will bubble or move water during power outs.
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400 gal reef. Established April, 2007. 3 Sequence Dart, RM12-4 skimmer, 2 x OM4Ways, Yellow Tang, Maroon Clown (pair), Blonde Naso Tang, Vlamingi Tang, Foxface Rabbit, Unicorn Tang, 2 Pakistani Butterflies and a few coral gobies
My Tank: http://www.canreef.com/vbulletin/showthread.php?t=28436
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