The fish was most likely sick. If the other fish in the tank are ok you most likely don't have any issues. It is common for new fish to die in the fish few days of being introduced into a new fish tank. There are many reasons for this to happen including PH balance in the tank, the water temperature could have shocked the fish or the fish was sick just to mention a few reasons why newly introduced fish die. I have had this issue many time in the past . Bottom line is try again. I believe if the other fish are fine you have nothing to worry about.
Take your dead fish and a separate sample of the water in the tank.
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