Hi 12cats. I hope your fish are doing better. I briefly read all those suggestions and conclusions and most of them really didn't make much sense. People telling you to check for parasites/ flashing but it would never be a case in cold weather. Also people that saying that cold weather wouldn't have anything to do with your fish problem likely never experienced any cold weather problems with koi. I also thought the same for many years I had ponds till a few years ago I experienced very similar problem you're having with your fish and then after extensive research found out that koi can have something that called "sleeping sickness" in the winter.
However Sleeping sickness is the wrong word for it, as it isn't a sickness but a problem that is described as behavioral abnormality.
They lie on the bottom of the pond looking to all intents and purposes to the world that they are dead, you'll find that even the gills will barely move. What has happened is that the cold has dramtically reduced their metabolisms to such a degree that they appear dead.
It usually happens to younger koi: in my case most of mine that were affected were 2 year old. "Sleeping sickness" is a thing that affect koi when temperature drops very quickly and their immune system goes very low in the short amount of time and they loosing their ability to normally function and lie on their sides. one of my koi started to do it back 5-6 years ago in january when temperature suddenly dropped. then all of the sudden i noticed him to start swimming fast what was very strange in cold water and after an hour of fast swimming he just collapsed on it's side on the bottom of the pond and looked dead. I decided to take him out to see if he's still alive and he was barely alive at that point. I took him in: set up the quarantine tank in the garage and let the water slowly warm up. Also gave him a baytril (Enrofloxacin) injection. He was still lying on his side in the tank but by the evening in about 5-7 hours later when water temperature reached about 50F he woke up and started to act normal. I didn't know what it was then kept him in the garage for 2-3 weeks, then we had some really warm days that winter what water in the pond reached over 50F and i decided to put him back. I got lucky with him and he hadn't have any problems for the rest of the winter. But about a month later 2 of my other koi started to have the same symptoms so i took them both in the garage and did the same as with the first one, they both recovered well and then I decided to put them back also, but that was a mistake as i found out later that it's best to keep this fish inside till spring before putting it back to the pond. So i lost one of the affected koi a couple of weeks later after i put it back in the pond. He was fine after warming him up and I only put him back when it was warmer out and water was closed to 50F, however he relapsed after it got colder again, and me bringing him in the second time didn't help anymore as likely his immune was too far gone from the stress of cold. you can read my post that i posted then and see most of people responses: https://www.gardenpondforum.com/threads/barely-alive-fish.10941/ One of the first fish that was affected i still have now alive and well and it never happened to it ever since. Even now I have one of the koi also lying on his side sometimes. It started back in november when temperature dropped suddenly below freezing. he started to show minor symptoms. This koi is way over 2 year old: he's 4 or 5 y.o. by now but apparently it could also occasionally happen to older koi. But his symptoms are very minor now, and i do still need to winterize pond which i'll do shortly. For the past 5 years i was running my pump and main filter all year around just in the winter i do shut off all the waterfalls and water just runs thru special winter pipe that i set up. and I put pipe heat wire on the pipes and also we build an insulated shed for all my pond equipment, and it seems to help tremendously to keep pond water warmer along with 3 deicers that i usually install but only turn them on in extremely cold weather. The reason that it helps because the external pump generates heat while it's running and by it staying inside insulated shed it keeps the filter warmer and warms up the water that funs thru it. Plus it also helps to jump start the pond in the spring as the filter wasn't dormant and all the good bacteria just hibernate in the winter and wakes up in the spring, and you still have mature filter instead of starting over with sterile one in the spring. I still have to turn off waterfalls and install deicers this winter and hopefully it'll help this koi with slight "sleeping sickness problem. In the attached photo you can see pond set up inside the shed. I hope it helps. Good luck and hoping you can save your koi thru the winter.
