Special autumn diet: why now is the time to change your fish food

At the end of summer, people often continue feeding their fish exactly as they did in July: the same food, the same quantities, out of habit. Then one October morning, they notice that the pellets are still floating an hour after feeding, the water is becoming slightly cloudy, and the koi are eating without enthusiasm. Autumn is when fish needs change most rapidly: they must build up their reserves for winter just as their digestion slows down. That is the whole point of special autumn food, and now is the time to make the transition.

What happens inside the fish's body in autumn

A koi carp or a goldfish does not regulate its temperature: its entire metabolism follows the temperature of the water. In summer, warm water allows rapid digestion and sustained growth; protein-rich foods are then used efficiently. In autumn, the same food remains in the intestine much longer. Digestive enzymes work more slowly, transit takes longer, and some of the food is expelled without being properly assimilated.

At the same time, the fish has a very concrete need: to enter winter in good condition, with sufficient reserves to get through several months of eating very little or not at all. A thin fish at the end of October faces winter with little leeway and emerges weakened in spring, at a time when parasites and bacteria take advantage of the slightest weakness, as described in fish pathology literature.

Autumn feeding must meet both of these requirements at once: provide proper nutrition, but with food that the fish can still digest.

What distinguishes an autumn food

Cool-season foods most often rely on wheat germ, known for its digestibility, with a more moderate protein content than growth foods. They are formulated to be assimilated more completely at low temperatures, which has a direct effect on the water: less nitrogenous waste, less organic matter at the bottom, and less work for the filter. As Boyd and Tucker point out, the filter's nitrifying bacteria also slow down considerably as the water cools.

Continuing with protein-rich growth food until November means combining slow digestion with a slow filter: the classic recipe for autumn nitrite spikes.

When and how to make the transition

The calendar is not the right guide: the water temperature is what matters. Here are the approximate ranges we use in the store.

The transition takes one to two weeks, replacing a little more of the old food with the new one each day. Fish do not like sudden changes, and neither does their intestinal flora. Keep a thermometer in the pond: a week of warmer weather may justify feeding a little more again, while a cold snap means reducing the amount immediately.

Common mistakes

First mistake: increasing portions “so they can build up reserves.” A fish does not store what it cannot digest; the excess ends up in the water. Reserves build up over time, with suitable meals, not by overfeeding fish in October.

Second mistake: relying on appetite. A koi that comes to the surface asking for food is not necessarily hungry in the sense we understand it: it is a learned reflex. Trust the temperature and the simple five-minute rule: anything not eaten within that time was too much.

Third mistake: keeping an opened bag since spring. Food that has been open for a long time loses some of its qualities, especially its vitamins. For autumn, a small fresh package is better than a large bag left lying around.

And to support their defenses?

Autumn is a sensitive period for fish health because their defenses slow down as the temperature drops. Good nutrition is the first foundation. The mucus covering their skin is their first barrier against parasites: some supplements are designed to help maintain it during transition periods. They do not replace healthy water or suitable food, but they can complement them.

Also read

The logical follow-up: Should I continue feeding my fish in winter?.

READ THE ARTICLE

Frequently asked questions

— At what temperature should you switch to autumn food?

As a general guideline, when the water remains below 15 to 18 °C, making the transition gradually over one to two weeks.

— Why wheat germ?

Because it is known to be very digestible. Combined with a more moderate protein content, it is better suited to digestion slowed by the cold.

— Should you feed them more before winter?

No. You need to give them suitable food in quantities the fish can digest. Overfeeding fish does not build reserves; it only creates pollution.

— Is autumn food also suitable for goldfish?

Yes, provided you choose a pellet size suited to their mouths. The principles are the same as for koi.

In summary

• In autumn, fish need to finish building up their reserves while their digestion slows down.
• Wheat germ food, which is lower in protein, is easier to digest and puts less strain on the filter.
• The transition depends on the water temperature and should be gradual.
• Reduce the quantities as the water cools, then stop.

Last updated: September 29, 2026.