If there was no parameter fluctuation, it must have been contaminated. In the middle of the night no less, -smacks forehead-. I think I would put my fish through a very quick drip acclimation over to tap water if they were dying that fast and I had no other option. Something is killing them fast in there and it may be your only option for saving them. Try to get them settled in buckets until you can get some RO from somewhere else and acclimate them back. I wouldn't put more than one least expensive fish back in the tank at a time after you get all the water replaced, you may end up having to nuke the whole thing depending on what got in there.
I would seriously take the rest of that water and my dead fish to the LFS and ask for some major store credit.
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