I am stupid Lazy did water change with tap water . everybody else in tank surived .> What could have been in the water????? Runnig carbon & poly-filter. I am so mad at myself!!!!!!
Maybe your fish was use to the dirty water.Many years ago i gave my neighbor an Undrulated trigger because it was creating havoc in my fish tank. Anyway he never changed the water until one day i couldn't stand it anymore ,i took it upon myself and made a water change on his behalf .Do i need to say more ?The damn thing died soon after.You might never know for sure why your fish died ,some tap water can have so much chlorine in it you can smell and taste it .How much water did you change from what size tank?Was the PH/Temperature the same as your tank water?When was the last time you changed the water?
When I was a kid my dad had a big fish tank for 25 years. His fish including many tangs never died, he never heard of using anything but tap water which he let sit for 24 hours prior to mixing with salt. He had a simple hang on back filter with poly in it that we cleaned out when we did water changes. He used dolomite and an undergravel filter with a couple power heads on each end of the tank
My point is that I highly doubt it was the tap water.