Do you add any chemical buffering agents to your water? Sometimes when these chemicals, or similar, are used in a tank with hard water then the calcium carbonate can precipitate out of solution and appear like a white dust. If this is the case then the cause of death could still be down to water quality whereby the parameters are not suited to the fish.
You state your levels as fine, but what were the actual figures, there may be a clue in there still
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