Koi carp (Cyprinus carpio) are among the most popular pond fish worldwide. Their robustness and longevity make them fascinating companions – provided they and their habitat are properly prepared for the cold season. Although koi are quite hardy, mistakes in pond care or improper feeding can have serious consequences. This article explores the biological basics and offers practical tips on how to optimally get your koi population through the winter.
Biological basics: What happens to koi in winter?
Koi are cold-blooded animals. Their body temperature adapts to their environment, meaning their metabolism, digestion, and activity levels are significantly reduced in winter. At water temperatures below 10°C, koi almost completely stop eating and enter a kind of hibernation.
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Metabolic rate decreases → lower energy consumption
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Immune system weakens → increased susceptibility to parasites and infections
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Oxygen demand remains → sufficient water quality remains crucial
Step 1: Pond preparation in autumn
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Remove leaves and organic residues
Fallen leaves and dead plant matter decompose in the water, producing foul gases (methane, hydrogen sulfide), and polluting water quality. A leaf net over the pond or regular removal of the leaves prevents problems. -
Check water quality
Nitrite, ammonium, and pH are particularly important. Elevated nitrite levels are toxic, while fluctuations in pH cause stress. A stable pH between 7.0 and 8.5 is optimal. -
Pond cleaning
Sludge on the bottom should be reduced before winter, as it produces fouling gases. A pond vacuum can be useful here.
Step 2: Proper feeding in autumn
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Temperature-dependent feeding :
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Above 15 °C: protein-rich summer feed
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10–15 °C: special easily digestible wheat germ feed
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Below 10 °C: no more feeding
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Background: Proteins are difficult to digest at low temperatures and can rot in the intestines – which further weakens the fish.
Step 3: Technical measures at the pond
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Ice protection
The pond should never freeze completely. Open water surfaces are necessary to allow foul gases to escape and oxygen to enter the pond. An ice-preventing device or a small pond aerator are recommended here. -
Pond filter
Many filter systems can be shut down in winter if the temperature drops consistently below 8°C. For larger populations or mild winters, reduced operation may still be advisable. -
Water depth
To ensure koi overwinter safely, the pond should be at least 1.5 m deep. In deeper layers, the water remains stable at 4 °C—the ideal retreat for the fish.
Step 4: Check during the winter
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Check regularly whether the water surface remains partially free of ice.
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Do not disturb fish from hibernation, as this consumes energy.
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If you notice unusual behavior (e.g. fish swimming apathetically at the surface), check the water parameters immediately.
Step 5: Get fit for spring
As water temperatures slowly rise in spring, feeding should be resumed gradually, starting with easily digestible food and then progressively increasing the protein content. At the same time, now is the best time for a thorough pond inspection and any necessary technical maintenance.
Conclusion
The successful overwintering of koi depends on three factors: good pond hygiene, proper feeding management, and technical protective measures against oxygen depletion and foul gases. By observing these aspects, you lay the foundation for the fish to start the new season healthy and vital in the spring.