If your koi fish have suddenly stopped eating, it can be worrying. But don’t panic just yet. Koi not eating is a common issue, and in most cases, it can be fixed once you find the cause. This article explains the main reasons why koi stop eating, how to identify the problem, and what you can do to help your fish return to good health.
1. Water Temperature Is Too Low or Too High
Koi are cold-blooded animals, so their body functions change depending on water temperature. When the water is too cold or too hot, their metabolism slows down and they may stop eating completely.
- Below 10°C (50°F): Koi naturally stop eating. Their digestion slows to a near halt.
- Above 30°C (86°F): High temperatures cause stress, lower oxygen, and a reduced appetite.
What to do: Use a pond thermometer to check your water temperature. If it’s too cold, don’t feed your koi at all. If it’s too hot, improve aeration and shade the pond. Only feed koi when water is between 15°C and 25°C (59–77°F).
2. Poor Water Quality
One of the most common reasons koi stop eating is bad water. High levels of ammonia, nitrites, or low oxygen can make koi sick and sluggish. Dirty water can even cause chemical burns to their gills and skin.
What to do:
- Use a water test kit to check for ammonia, nitrites, nitrates, pH, and oxygen levels.
- Change 20–30% of the pond water if levels are too high.
- Clean filters and remove decaying leaves, waste, or uneaten food.
- Add aeration — use air stones, fountains, or a venturi system to raise oxygen.
3. Recent Changes or New Environment
Koi get stressed easily, especially after a big change. Moving to a new pond, adding new fish, or even a strong storm can cause them to stop eating for a few days.
What to do: Give your koi time to adjust. Avoid handling them or doing more changes. Keep the environment stable, and try feeding small amounts once or twice a day until they start showing interest again.
4. Disease or Parasites
If your koi aren’t eating and also look sick, there may be a health problem. Look out for these signs:
- Clamped fins or hiding at the bottom
- White patches, ulcers, or cloudy eyes
- Scratching or flashing against objects
- Gasping for air at the surface
These could be signs of bacterial infection, parasites like flukes, or fungal issues.
What to do: If you notice physical symptoms, isolate the sick fish if possible. Consult a vet or experienced koi keeper before treating. Avoid over-the-counter medication unless you’re sure of the issue — treating the wrong problem can make things worse.
5. Food Issues
Koi may ignore food if:
- The food is stale, expired, or has gone moldy
- The pellets are too large or too hard
- The food sank too fast or blew away on a windy day
What to do:
- Check the expiry date and smell of the food
- Store food in a cool, dry place
- Try soaking pellets in pond water for 30 seconds before feeding
- Use smaller floating pellets or a feeding ring
6. Breeding Season Stress
During breeding season (usually spring and early summer), males often chase females aggressively. The stress of this activity can cause koi to stop eating temporarily.
What to do: Provide hiding spots and water plants. If one fish is being bullied, consider moving it to a separate holding tank until the activity slows down. After spawning, koi usually return to normal behavior.
7. Overfeeding and Leftovers
If you’ve been feeding too much, koi may simply not be hungry. Leftover food can also rot and damage water quality, which leads to appetite loss.
What to do:
- Feed only what they can eat in 3 to 5 minutes
- Feed once or twice a day when water is warm
- Skip feeding for a day or two to reset appetite
8. Oxygen Problems
In warm weather, oxygen levels in water drop. If koi seem slow, are gathering near waterfalls or air stones, or are gasping, they may be suffering from low oxygen.
What to do: Add more aeration. This can include a larger air pump, a waterfall, or a venturi pipe. Also, reduce stocking levels if your pond is overstocked.
9. Chemical Exposure
Accidental chemicals, such as chlorine from tap water or fertilizers from nearby lawns, can poison koi and affect their appetite.
What to do:
- Only top up ponds with dechlorinated water
- Use pond-safe water conditioners
- Keep garden chemicals away from the pond edge
10. Old Age or Seasonal Behavior
Older koi may eat less than young, active ones. In early spring or late autumn, appetite may naturally drop due to changing temperatures.
What to do: Observe each fish’s behavior. Switch to easily digestible wheat germ food in cooler weather. Don’t overfeed during temperature swings.
Quick Checklist: Why Your Koi Might Not Be Eating
| Cause | What to Check | Solution |
|---|---|---|
| Cold water | Water temp below 10°C | Stop feeding until warmer |
| Poor water quality | Ammonia, nitrites, or pH off | Test water, do a partial change |
| New environment | Recent move or additions | Let them settle, avoid stress |
| Disease or parasites | Visible signs or flashing | Quarantine, seek diagnosis |
| Bad food | Stale, moldy, or expired | Replace food, feed less |
| Low oxygen | Fish gasping or sluggish | Add aeration, clean filters |
| Breeding stress | Chasing or hiding behavior | Provide shelter, reduce pressure |
How Long Can Koi Go Without Eating?
Koi can survive for 1–2 weeks without food, especially in cool water. During winter, they may go months without eating, and that’s completely normal. A healthy koi can go a few days without food while you figure out the issue — just make sure water conditions are safe and stable.
When to Worry
If your koi:
- Haven’t eaten in over a week during warm weather
- Show physical signs of illness or injury
- Are acting unusually (floating, gasping, isolating)
…then it’s time to take action. Reach out to a koi expert, pond specialist, or veterinarian who can guide you on testing and treatment options.
Conclusion
Koi not eating can be caused by something as simple as cold water or as serious as a parasite infection. Start by checking the water quality and temperature, watch your fish closely, and look for changes in behavior or appearance. Most causes are manageable, and koi usually recover quickly when the issue is solved. With a little observation and care, your koi will be back to feeding normally in no time.