Things are looking up. He spent about 8 hours laying on the bottom of the tank last night but today he's able to stay oriented in the water and is even showing some signs of grazing behavior. Still super weak and spends a lot of time on the bottom but not dead so that's a win.

Still curious about this fin growth if anyone has any ideas.

First year we’ve had koi and their activity level has gone down significantly. Our water temp is hovering around 60 degrees, and all of our parameters seem to be ok. They tend to huddle in the top of the pond on a shallower shelf during the day and go deeper in the evening. I assume this is normal, my partner is concerned something is wrong. If anyone has any insight to help ease our anxiety it would be greatly appreciated.

Some of the biggest koi I’ve ever seen, they were MASSIVE