I'm so sorry to hear this happened.
I, too, would like to understand better. After all, fish live in natural ponds and lakes in the same climate, and survive the winters. Does it have to do with oxygen? temperature? toxins?
Zoe, why did you salt your pond for winter?
Cath, when you get your new fish, be sure the water quality is good before introducing any new ones. Maybe start out with a few 22c feeders from PetSmart, and be sure they thrive, before stocking up with pretty goldies and koi.