Perhaps I read wrong, but one thing that struck me is that you said all the fish are dying/dead but the corals all remained fine?
I've had SPS corals survive out of a bag that reeked of death and were probably off the chart in ammonia - but fish would have been dead in that water for sure so maybe you had an ammonia burn or something similar due to stirring up crap, and also having a lower total volume (due to water displacement by the buckets of water). Maybe you had a dissolved oxygen issue also? I find that a lot of mysterious mass deaths in the night can usually be attributed to DO issues.
That being said, this is one of those "crashes" where the source is going to be difficult or impossible to pinpoint. But a better water change regimen would be a good place to start.
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