Hello Sid.Have you been feeding them throughout this cold weather? If so then this is one cause,as food will sit in their digestive system and rot in their gut,poisoning them.I never feed mine from about the end of October until it gets warm again in spring.
Otherwise all i can say is that fish do sometimes die for no apparent reason.Is your pond deep enough and large enough for the number of fish you have? Also is it well maintained,ie filtered for the long cold winter ahead,where they spend the next few months at the bottom of the pond,which needs to be reasonably clean.?
