According to this story our Koi get drunk in the winter!
 
During the winter, goldfish and their relatives can have a blood-alcohol concentration beyond the legal limit for drink driving.
Scientists have figured out how goldfish produce alcohol to survive when trapped beneath the icy surfaces of frozen lakes and ponds.Unlike most vertebrates which die within a few minutes without oxygen, goldfish and their wild relatives crucian carp are able to survive for months in low oxygen water.
 
https://news.sky.com/story/goldfish-go-over-the-drink-drive-limit-to-sur...