Sorry to hear that. :'(
I'm no expert, all I have to say is from my pond experience. Unfortunately your new fish may have been carrying a disease or it may have caught something from the oldies in your pond that it is not immune to certain "schtuff".
Either way I have had fish die with all the care I could give them before. Now I buy from a reputable clean dealer and make sure the fish is really really healthy looking before even bringing it home to the QT pond. Only my DD makes me release the fish from QT earlier than I should. So far since the early ugly days long ago I've been doing A ok.