sorry to hear of your loss.
you said the water was cloudy that morning,and youve now idea what caused the clouding of the water.
just a guess .
if it was a dodgy fish that infected the tank then that could explain the deaths if the water tests are reading ok.sometimes a fish will skip along the sand/gravel and kick up the dust or dirt before dying,bit like a spasm if you like.
if the infection had spread to the other fish allready then they would be taking in that dirt and this could have started the ball rolling.next one does the same and so on.hence cloudy water and dead fish.
just a guess.