Autumn is the season that defines your koi’s health for the next few months in winter and spring. As temperatures cool and metabolism slows, small adjustments to feeding, filtration, and aeration will make the difference between thriving koi and winter stress. This Fall 2025 guide breaks down exactly what to do—by temperature range, not by calendar date.
1. Early Fall (70–60 °F / 21–16 °C): Transition Season
- Feeding: Mix your summer diet with wheat-germ-based food for easier digestion. Feed small meals once or twice a day, finishing within 3 minutes.
- Maintenance: Remove debris and trim plants before leaves begin to drop. Clean mechanical filters weekly; rinse bio-media only in pond water.
- Testing: Check ammonia, nitrite, nitrate, KH, and pH weekly.
- Observation: Watch for parasites or ulcers—fall warmth gives you one last chance to treat safely.
2. Mid Fall (60–50 °F / 16–10 °C): Gradual Slowdown
- Feeding: Switch fully to wheat-germ food. Reduce to one meal per day at 55–60 °F, then every other day as you near 50 °F.
- Water Quality: Do 10–15% partial water changes every 1–2 weeks with temperature-matched, dechlorinated water.
- Aeration: Add an air stone at mid-depth to improve oxygen levels as biological activity declines.
- Equipment: Bypass waterfalls to reduce cooling. UV sterilizers can be cleaned and stored once algae growth stops.
3. Late Fall (50–40 °F / 10–4 °C): Winter Prep Mode
- Stop feeding when water temperature drops below 50 °F—koi cannot digest food efficiently below this point.
- Final cleanup: Remove all floating debris, fallen leaves, and decaying plant matter. Clean filters one last time.
- Gas exchange: Use an aerator or de-icer to keep a single ice-free hole. Avoid disturbing the warm bottom water layer.
- Predator defense: Add pond netting and motion-activated deterrents.
4. Key Tips for Koi Keepers
- Feed by thermometer, not by habit. Every koi pond behaves differently—watch the temperature, not the calendar.
- Maintain stable pH and KH. Fluctuations in these parameters stress koi more than cold itself.
- Clean, but don’t over-clean. Preserve your biofilter bacteria so it can rebound in spring.
- Plan aeration before freezing hits. Mid-depth bubbling preserves oxygen while protecting the warmer lower layer.
5. Fall-to-Winter Checklist
- ✔️ Thermometer installed and monitored weekly
- ✔️ Netting in place to catch leaves
- ✔️ Feeding tapered off and stopped ≤50 °F
- ✔️ Aeration and de-icer positioned at correct pond depths
- ✔️ Final water test before winter sets in
Fall isn’t the end of pond season—it’s the foundation of the next one.