Enter winter with the pond already clean and stable. A very old koi that looks thin, moves slowly or feeds weakly in autumn carries little reserve into cold water. Cold water slows digestion, movement and immune response. Your job is to remove work from the fish before temperatures fall and then to disturb the pond as little as possible until spring.
Prepare the pond before the cold arrives
Start six to eight weeks before you expect sustained cold. Skim and net out fallen leaves and clear sludge from shelves and corners. Decaying matter uses oxygen and releases ammonia through winter. Test ammonia, nitrite, nitrate and pH, and correct any problem with partial water changes matched for temperature. Check pumps, air lines and filters while you can still work comfortably with wet hands.
Look at each old fish closely during this period. Note which fish hold a rounded body line and which fish show a pinched belly or a prominent backbone. Photograph thin fish from above beside a familiar object for scale. A fish that enters winter thin needs the calmest routine and the closest watching. Do not add new fish in autumn, because newcomers bring extra waste and extra activity at the wrong time.
Feed a wheatgerm-based diet in the cooling weeks if you feed at all, and offer smaller meals only while fish seek food eagerly. Stop feeding once water stays below 50°F (10°C). Below that point digestion slows sharply. Uneaten food and undigested food both foul water that you want to leave undisturbed.
Set up shelter, depth and oxygen
Old koi rest in the deep zone where temperature changes slowly. Confirm that the pond gives at least one area that stays well below any ice layer and away from waterfall splash and strong return flow. Turn off or reduce features that chill and stir the whole pond. A gentle air stone in the deep zone or a floating de-icer that holds a small hole open lets gases escape without creating strong current.
Cover part of the pond with a net before leaf fall peaks. Leave the cover in place through heavy leaf weeks and lift sections only for checks.
Avoid netting, catching or moving frail koi during winter unless you see an injury or a fish trapped in shallow ice. Handling in cold water costs energy and damages the protective slime coat at a time when healing runs slowly.
Watch through winter without interfering
Visit briefly two or three times a week. Look through the ice hole or from the edge on mild days. Resting near the bottom with fins held loosely is normal for cold koi. Call for aquatic veterinary advice promptly if you see a fish lying on its side, gasping at the hole, showing sores or fuzzy patches, or trapped in ice. Describe what you observed and share your autumn notes rather than guessing at a cause.
Do not break ice by striking it, because shock waves travel through the water. Pour warm water gently to reopen a frozen hole.
Restart feeding with extra care in spring
Wait until water holds above 50°F (10°C) for several days and fish begin to move and look for food. Offer a tiny wheatgerm meal first and remove anything uneaten within a few minutes. Thin fish get priority watching at these meals. A fish that ignores food for a week after others feed, loses more weight or develops marks on the skin needs veterinary input before you increase feeding for the pond.
Resume water testing weekly during the restart, because filters wake up slowly and waste rises fast once feeding returns. Increase meal size over two to three weeks, not over two to three days. Keep notes on which old fish eat, how they swim, and how they hold their body line. Those notes turn a vague worry into a clear history. KinsLe Pet Passport gives that history a tidy home, so ages, autumn weights in words and photos, winter readings and spring feeding notes travel together to any vet or pond professional who helps you.