I have no idea and not a lot of information to go on. Without a necropsy, you can't say for sure what killed him or them.
On Long Island, Zone 7, I had my goldfish and koi go foodless from roughly mid October through April 1st. And when I started to feed, it was with fortified wheat germ pellets. As they awaken from their winter slow down, that's all my fish could digest. Their immune systems are surpressed. I did not feed a regular diet until the water temperature reached the mid 60's.
I followed the reverse process in the fall and by mid to late September was feeding wheat germ pellets again. When the water went below 50 degrees Fahrenheit, I stopped all feeding.
Once the ice was off the pond I had the biological filter cranked up. The UV filter ran from late March until early November. Then I remove the bulb and stopped the circulating pump. The UV filter requires a new bulb every year.
