damn shame ... sorry to hear your loss ...i think there wasn't enough airation in the water after transfering ... whatever setup you had it before, they were used to it .. after transfering, they were suffocated bc of lack of oxygen
i've lost several prize fish in 8 hr time after a water change before realizing that water level was above spray bar and ones that were still alive were gasping for air ... the dead ones died of suffocation
.. life goes one .. hope you're already looking into getting new fish ...