Its getting colder by the day, in a couple of weeks its gonna be dark at 5pm and it will probably be April before daytime temps are similar to those we currently have.
Real depressing, but you cant beat nature.
Few scribbled thoughts;
Temperature -
Koi are pretty hardy but are not cold water fish, when the water temps drop below 5C you are running a tightrope and when it gets sub 2C it’s touch and go. Yes you will hear Guys saying that their ponds have been that low and lower without any casualties but for every one there are 20 untold stories to the contrary.
Some heating facility, even if simply to prevent temps dropping sub 5C will take away a load of risk. Yes there is a cost but in reality we are only talking of it being needed for a very small time frame of just a few weeks in the depths of winter.
If it gets real cold and pond temps are critically low then you may find that the supply water is warmer than the pond so adding more water helps increase the pond temp.
For all Koi keepers I would suggest you want to consider an emergency plan just in case we have a real exceptionally cold spell.
For Guys that control temps then it’s really down to personal preference and depth of wallet as to what temps they keep the Koi at through the winter. There’s a balance between how effective the immune system of the fish is and how active any parasites development may be and it’s probably something for discussion in a separate thread .
Covers
A pond cover will help reduce heat loss and if you are unheated it can make that small difference and want to do something to help avoid casualties then this is the first step.
Polycarb is ideal, but bubble wrap on a frame serves a very useful purpose in stopping wind chill. One thing to bear in mind is that there does need to be some ventilation to allow gasses to escape.
U values give an indication of thermal efficiency of different materials, something to consider when weighing up which cover to go for if you are heating.
Koi need light, if not - there is risk that beni can be affected - so ideally the cover should be translucent.
Filters
Keep them running, clean them once a week. Good husbandry is important, the fishes immune systems are gonna be low at lower temps so you don’t want to allow muck to build up through neglect.
Just remember that filters aren’t gonna be doing much Bio wise when temps get below 14C.
Air
There are different views on this, some reckon on turning it off as it messes with the thermoclines in the water, the warmest is at the bottom.
My take is that in the depths we are talking about this makes no difference but I tend lift aerators higher up to give the fish somewhere to rest.
Water changes
I would keep up a minimum of 10% a week.
Remember to put the hose away after so it doesn’t freeze.
Protect any carbon filters from freezing!
Behaviour
When it gets cold the Koi will spend a lot of time on the bottom. Take a little care not to suddenly stress them unless there is a real need.
Feed
Personally when the temps get below 10C I don’t feed, others will throw in a small amount of wheat germ if the fish are active - what you want to avoid is on a cold bright sunny day suddenly throwing in a load of food.
Medication
Hopefully you will not need it but take particular care in the colder months, its even more important to be absolutely sure of the target as at lower temps any medication will knock the fish back.
The good thing is that the critters don’t thrive in the cold so if the fish are clean and healthy going into the winter there shouldn’t be too many problems.
Water Testing
For the Guys in soft water areas then the need to keep checking they have some Kh in the water remains, all to easy to get complacent and end up crashing the pH.
For the rest weekly ammonia and nitrite as usual.
Dave
Winter thoughts 
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