The danger under a sealed surface
A koi pond in winter is a slow, cold, breathing system, and ice threatens the breathing. Once a complete sheet seals the surface, gases from rotting leaves and waste below can no longer escape, and fresh oxygen can no longer enter; in a long freeze the water under the ice grows steadily worse while the fish sit torpid in the coldest layer, unable to do anything about it. Koi tolerate cold well — it is the sealed, stagnant water that harms them. The whole of winter preparation is aimed at one thing: never letting that seal become complete and airtight for long.
Clear the pond before the cold sets in
The work starts while the water is still mild. Net or skim out fallen leaves before they sink, cut back and remove dying pond plants, and lift out the soft sludge of the season from shelves and the pond floor where you safely can. Everything left to rot under ice becomes gas under ice, so an hour of clearing in autumn is worth more than any winter gadget. A net stretched over the pond during leaf-fall catches the bulk of it and is the single easiest winter preparation there is. Check the net weekly and empty it before the weight of wet leaves drags it into the water.
Keep a breathing hole in the ice
Arrange the open hole before you need it. A floating pond heater or de-icer holds a small circle of water open even in hard frost and is by far the most reliable method; a pump or airstone running just below the surface keeps water moving and delays freezing in milder spells. If ice forms without either in place, melt a hole with a pan of hot water rested on the surface — never smash the ice, because the shock waves travel straight through the water to the fish and can injure them in their torpid state. Recheck the hole after every cold night; a hole that froze over by morning is a routine fix, not a failure.
Feeding stops as temperatures fall
Koi digestion slows with the cold until it effectively pauses, and food eaten in near-freezing water can sit undigested and cause real harm. Shift to a wheatgerm-based cold-water food as temperatures slide in autumn, then stop feeding altogether once the water is properly cold and the fish have stopped coming up looking for food — they will live on their reserves until spring, as they are built to do. Uneaten food in winter is doubly bad: it fouls water that cannot exchange gases freely. Resist the mild-day temptation to feed; one warm afternoon does not restart a winter metabolism.
A winter checklist that runs itself
Sequence the season so nothing depends on remembering in the cold: leaves netted and cleared, plants cut back, de-icer tested and floated before the first freeze is forecast, pump and filter dealt with according to their type, feeding tapered then stopped, and the hole checked through every freeze. Written as a checklist, the same jobs recur every year with the guesswork removed, and anyone else looking after the pond in your absence can follow the same steps in the same order. Putting those ice-season jobs into KinsLe's Care Teams means each winter task is assigned, dated and marked done before the freeze arrives.