Hard to diagnose. The only thing I can come up with is ammonia poisoning. I realize your ammonia level was zero, but it will always be zero because ammonia gets converted very fast by bacteria.
The tank is large enough so that should not be the cause but it is a very new tank and ammonia poisoning causes the fish to die with their mouth wide open although I can't tell by the picture if that is the case. That is usually the case in a new tank because even though you cycled the tank
that only means the tank has the capacity to remove ammonia from whatever you cycled the tank with and nothing more. Any new fish added would need time to build up enough bacteria.
Not days but possibly months in such a new tank.
A tank continually "cycles" through the life of the tank with the bacteria reproducing and waning as the organic load varies.
Of course this is a guess as I can't see your tank from here but if the rest of your fish are still fine, that is my "guess" for now.