Ammonia is a quick and effective killer. Makes sense to me that the ammonia will kill a fish at night when O2 levels are lowest. You should never be able to detect ammonia in a mature aquarium. Double check with a new test kit.
If you have verified ammonia level in the system, I don't see any point in chasing any other problem. If present, it indicates that the bacteria of the aquarium has been overwhelmed by something rotting, or by a cleaning you've done recently.
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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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