Koi Behaviour: Feeding, Resting, Chasing and Warning Signs
Get to know what is normal for your koi
One fish is always first to the food. Another waits until you step back. On a cold morning the whole pond may seem quiet, then a few weeks later the fish are following you along the edge. Spending time watching them makes those differences familiar.
The useful part is noticing a change. A normally keen feeder hanging back, several fish suddenly crowding a return or repeated rubbing against the pond wall deserves a closer look. Calling a fish shy, grumpy or playful does not tell you whether the water is suitable.
We enjoy the character in a pond as much as anyone, but we would separate what we can observe from what we are guessing. This guide explains common koi behaviour, the checks behind an unusual movement and when to get help.
Watch before the food goes in
Stand back for a few minutes before approaching the usual feeding point. Food changes the scene: fish gather, compete and swim near the surface, which can hide what they were doing a moment earlier. A quiet look first is particularly valuable if something seems different.
Look across the whole pond, then follow individual fish. Are they upright and swimming evenly? Are their fins held as usual? Is one fish staying apart? Can you see an obvious injury, raised scales or a change around the mouth or eyes?
Notice breathing as well as swimming. Compare a fish with its usual pattern and with other fish in the same water. There is no single useful gill-movement count for every koi at every temperature. Rapid, laboured or one-sided breathing is more significant than trying to match a number found online.
Check again from another safe viewing point if reflections hide part of the fish. A viewing window can make the sides and underside easier to observe, although glass and clear water do not replace proper examination. Our guide to shy and hiding koi discusses observation and helping fish settle.
A short video is useful when a movement is difficult to describe. Record a normal stretch as well as the unusual moment, showing the other fish if possible. Keep the date, water temperature and recent events with it. Do not chase the fish to reproduce the behaviour for the camera.
What everyday koi behaviour can look like
Koi spend time exploring, taking food, moving around the pond and resting. The balance changes with temperature, light, disturbance and the individual fish. Watching the same pond regularly is more useful than comparing a few seconds with a lively feeding video from somebody else’s pond.
| What you notice | Context to consider | What deserves attention |
|---|---|---|
| Fish gather as you approach | They may associate the place or routine with food. | Gulping continuously or struggling to breathe, rather than simply taking pellets. |
| A fish hangs back from feeding | New surroundings, temperature, competition or an individual problem. | A change from its usual appetite, repeated refusal or visible injury. |
| Fish rest quietly lower down | Cold water, time of day and normal resting patterns. | Loss of balance, lying on a side, persistent clamping or weak response. |
| Fish chase and splash | Spawning can involve vigorous chasing. | Exhaustion, trapped fish, injuries or abnormal breathing. |
| A fish rubs or flashes | Irritation needs investigation if it repeats. | Several fish affected, frequent rubbing, skin changes or breathing trouble. |
| Fish crowd an inlet or air source | Look at flow, oxygen and water quality. | A sudden group change should be treated promptly. |
A group swimming together is not proof that the water is safe, and a fish spending a little time on its own is not a diagnosis. Look at the combination of signs and whether the pattern has changed. It helps to describe exactly what happened rather than begin with a disease name.
The Merck Veterinary Manual’s fish-health guidance includes changes in appetite, breathing, swimming and body condition among the signs that warrant attention. Those observations are a reason to investigate, not a way to identify a particular infection from the pond edge.
Following you, hand feeding and the fish that always wants more
A regular feeding routine can make fish much more willing to approach. The movement of the food container, your position and the time of day may all be part of that routine. It is enjoyable, but an eager response is not a reliable portion measure.
Watch the quieter fish as well as the first one to the pellets. Food can be eaten quickly by the most confident fish while another is receiving little. Spreading the same sensible portion across an accessible feeding area may help observation, provided it does not carry food into the skimmer or a dead corner.
Do not keep adding food until every fish stops showing interest. Use a measured portion and check that the food offered is eaten within about five minutes. Leftovers tell you too much went in. The filter should not be processing food you could have kept in the tub.
Matt’s own ponds have substantial filtration and biological media. His automatic feeder gives six feeds roughly three hours apart, with occasional additional hand feeding when he is outside. That is his established routine for those ponds. It should not be copied without looking at your fish, water temperature, food amounts and filter performance.
The koi food guide covers the foods and Matt’s personal choice. Our separate feeding routine guide explains portions and automatic-feeder checks. More opportunities to feed should mean controlled smaller portions, not six full meals on top of the previous daily amount.
When a keen feeder stops eating
Check the water temperature, ammonia, nitrite and pH, along with aeration and flow. Ask what changed: food, medication, new fish, a large top-up, a filter clean or disturbance around the pond. Look for mouth damage or other visible signs without repeatedly catching the fish.
If only one fish is affected, that is still worth taking seriously. If several stop together, the shared environment needs urgent attention. Do not use a favourite treat to persuade an unwell fish to feed while overlooking the reason for the change.
Resting on the bottom or becoming less active
Lower activity in cold water can be normal. Fish may spend longer in sheltered areas and show little interest in feeding. Compare the behaviour with measured water temperature and the usual pattern in your pond, not just how warm the air feels.
Resting fish should still be assessed in context. Upright fish quietly holding position are a different sight from a fish rolling, lying over, breathing heavily or unable to swim normally. Do not call every bottom-sitting fish dormant, particularly in warm water or when only one fish is affected.
Check for a recent equipment problem. A stopped pump, reduced air or blocked return can alter conditions without an obvious change in the water’s appearance. A clear pond can still contain an unsuitable ammonia or nitrite reading.
Persistent lethargy can have several causes. The University of Florida’s information on carp edema virus disease describes a sleepy presentation, while also explaining that similar signs can occur for other reasons. That is precisely why a name such as “sleepy disease” should not be assigned to a fish simply because it rests on the bottom.
Keep watching through winter without repeatedly disturbing the fish to see whether they will move. Our winter feeding guide and seasonal care guide explain how temperature changes the routine.
Are the koi fighting or spawning?
Spawning can look surprisingly rough. Fish may chase closely, push along another fish’s sides and splash around shallow areas or planting. Eggs may be visible afterwards. It is a different pattern from an occasional jostle around food, but a video and the wider context are more useful than one description.
Koi are carp, and egg release and fertilisation happen in the water. The University of Florida’s ornamental-fish overview describes group spawning and egg scattering among cyprinids. That background helps explain the activity; it does not make every chase harmless.
Watch for a fish becoming exhausted, trapped against equipment or repeatedly forced into a damaging edge. Look for injuries afterwards and check water quality, because spawning adds organic material to the system. Keep aeration working and avoid adding extra food simply because the pond is active.
Do not try to squeeze eggs from a fish or chase it around with a net to stop the activity. If a fish is being injured or cannot recover, seek experienced help with a safe plan for separation. An unfiltered container is not automatically a suitable place to put a stressed adult koi.
Finding eggs does not mean you need to raise every fry. Consider the space and mature filtration required before trying to rear more fish. Do not release unwanted fish or pond water into natural waters.
Flashing, rubbing and jumping
Flashing usually describes a quick turn or rub that exposes a flank, sometimes producing a flash of colour from the scales. A fish may scrape against the bottom, wall or equipment. Record how often it happens and whether the same fish or several fish are involved.
Repeated rubbing can indicate irritation. Water quality, recent changes and parasites are among the possibilities, but the movement does not identify which is responsible. “Flashing means flukes” is too simple a conclusion.
Start with current water results and the treatment history. Check whether the behaviour began after a top-up, a product application or equipment work. If it persists, proper examination and appropriately taken samples may be needed. A microscope result has to be interpreted with the fish’s condition and the wider pond information.
Do not medicate the whole pond repeatedly just because one occasional rub was seen. Equally, do not ignore frequent flashing, damaged skin or breathing trouble while waiting for a clearer symptom. Our koi health and diagnosis guide explains the next steps.
A koi jumping out needs prompt attention
A jumping fish can injure itself or leave the pond. Make the edge safe where possible, check water and equipment and investigate the cause. If a suitable temporary restraint is needed, it must be fitted safely without creating an entanglement or access risk.
Do not assume a jump proves happiness, play or a particular parasite. The timing and repetition matter, as do the condition of the other fish and any recent change. If a fish has left the water or sustained an injury, obtain appropriate help rather than improvising treatment.
Gasping, crowding the return and rapid gill movement
Continuous gulping at the surface is different from coming up to take food. Several fish gathering at a return or air source, especially with rapid breathing, should prompt immediate checks. The pond may look clear and the waterfall may still be running.
Check that suitable aeration and circulation are actually delivering water and air. A blocked inlet, failed diaphragm, kinked airline or power interruption may be obvious once you look. Keep essential support working while the cause is investigated.
Test ammonia, nitrite and pH and measure temperature. Low oxygen is one possibility, but gill irritation, nitrite and disease can produce similar outward signs. The RSPCA’s fish-health advice highlights persistent surface gasping and recommends checking water quality and seeking a specialist fish vet where diagnosis is needed.
If only one fish breathes abnormally, investigate that fish as well as the water. A group problem and an individual problem can overlap. A fish that improves after more air has still given you a reason to find out what went wrong.
For heavily stocked ponds, ask whether the air arrangement still suits the fish and feeding. The output a pump delivers falls as depth and resistance increase. See the air pump and diffuser guide before choosing equipment from pond volume alone.
The information that makes advice useful
“My water is fine” leaves out the most useful details. Write down the test names, actual results and units, the kit used and the time. Check the date and condition of reagents and follow the reading time. If an unexpected result appears, repeat the test correctly without delaying urgent support for distressed fish.
Single tests can be useful for repeating a particular check. We sell the NT Labs Pond Ammonia Test Kit and Pond Nitrite Test Kit, as well as the Pond Lab Multi-Test Kit. Check how the result is reported before comparing it with a different brand’s scale.
Keep a short history beside the results: when the problem started, whether one or all fish are affected, new arrivals, products used, water added and filter work. Photographs of product labels are more useful than remembering that a blue bottle went in last week.
| Observation | Useful detail |
|---|---|
| The fish affected | One individual, a few fish or the whole pond; size and visible identifying marks. |
| Timing | First noticed, frequency, duration, and whether it happens before or after feeding. |
| Water | Temperature, test results, units, kit and time. |
| Equipment | Normal versus current flow, air delivery, recent faults and cleaning. |
| Recent changes | New fish, food, treatment, top-up, weather or work around the pond. |
| Evidence | An unforced video of the behaviour and clear photographs of visible changes. |
Photographs help describe what you can see. Keep water-test results and the recent history beside them, because a still image cannot show every cause of a change in behaviour.
When to involve a fish vet or the Fish Health Inspectorate
Seek prompt professional advice for persistent breathing trouble, loss of balance, ulcers, rapid deterioration or unexplained deaths. Do not keep changing treatments while waiting for a diagnosis.
In England and Wales, suspected listed fish disease and increased unexplained mortality need reporting to the Fish Health Inspectorate. The current government KHV guidance explains the signs and reporting route. Do not move fish, pond water or shared equipment elsewhere while a serious infectious problem is being investigated.
Common questions about koi behaviour
How can I tell whether my koi are happy?
Look for the practical signs of suitable care: normal appetite for the temperature, steady swimming, comfortable breathing and sound body condition, supported by water tests. No single gesture proves an emotional state, and enthusiastic feeding does not rule out a developing problem.
Do koi recognise their owner?
Fish can learn routines and associations, and many keepers see a strong response to the person who feeds them. Enjoy that interaction without treating it as a water-quality test or proof of human-like affection. A consistent, calm routine is more useful than repeatedly testing how close you can get.
Why are the fish hiding after a heron visit or pond work?
Disturbance can change their willingness to approach. Check water and equipment first, then give them a calm routine and time to settle. Our hiding and shyness guide covers this in more detail.
Is one flash after feeding normal?
One brief observation does not establish a cause. Note it and check for repetition, water changes or other signs. Frequent rubbing, several fish affected or any breathing problem needs a proper investigation rather than an assumption that it was only excitement.
Should I catch a fish every time it looks different?
No. Start with observation, equipment and water checks. Handling has its own risks. If examination is needed, have the correct equipment and experienced help, rather than repeatedly chasing a fish with a general garden net.
For an everyday care routine that helps you notice changes early, use our keeping koi healthy guide. Knowing your fish well is useful precisely because it helps you spot when something is different and describe it clearly.
