Why Is My Koi Lethargic or Sitting on the Bottom

The short answer

A koi may become lethargic or sit on the bottom because of:

1. Low dissolved oxygen, especially before sunrise, after a hot day, during a power failure, or when organic matter is decomposing.

2. Ammonia, nitrite, chlorine, chloramine, unstable pH, low KH, or another chemical irritation.

3. Water that is unusually cold or hot, or a sudden change in temperature, pH, salinity or flow.

4. Stress caused by transport, handling, spawning, aggression, predators, construction, crowding or a recent treatment.

5. Gill or skin parasites, bacterial disease, viral disease, internal disease, or damage caused by a previous water-quality event.

6. A mouth, spinal, neurological or buoyancy problem that makes normal swimming difficult.

7. A short period of normal rest, particularly at night or after a minor disturbance, when the fish remains upright, responsive and otherwise normal.

One koi resting briefly is not automatically an emergency. A koi that remains on the bottom for hours, becomes difficult to rouse, breathes abnormally, loses balance, refuses food, separates from the group, shows pale or damaged gills, or is joined by other affected koi should be treated as a health warning.

Start with the pond. Check aeration and circulation, test the water, observe the entire group, remove obvious waste and reduce disturbance. Do not raise the temperature to “wake the fish”, add several medications together, or repeatedly net the koi before you know what you are trying to diagnose.

What lethargy actually tells you

Lethargy means that the koi is showing less spontaneous activity than is normal for that fish and that pond. It may swim slowly, remain in one area, react late to movement, or stop participating in the group. Bottom sitting is a location and posture, not a diagnosis. The fish may be resting on the bottom while remaining completely capable of swimming away, or it may be on the bottom because swimming has become physiologically difficult.

Fish are ectothermic, so their metabolism and muscle performance are strongly influenced by water temperature. Their gills must also work continuously to obtain oxygen, remove carbon dioxide, maintain acid-base balance and excrete nitrogenous waste. The kidneys, skin and mucus layer help maintain fluid and mineral balance. A problem affecting any of these systems can make normal swimming expensive in terms of energy.

This is why a koi that looks “lazy” may not be lazy at all. It may be reducing movement because its gills cannot deliver enough oxygen, because the water is irritating its tissues, because it is trying to conserve energy during a stress response, or because its muscles and nervous system are not coordinating properly. Behaviour is the visible end of several hidden processes.

The most useful first distinction is between a resting fish and a compromised fish.

A koi that is probably resting

A normally resting koi usually remains upright, keeps its fins in a relaxed position, breathes at a normal rate for the pond, and responds when you approach without panicking. It may move away smoothly, rejoin the group, or resume normal cruising after the light changes, feeding begins, or the disturbance ends. The body does not look sunken, swollen or unstable.

Resting behaviour is more likely at night, after a period of activity, after feeding, in cooler water, or after an unfamiliar noise. A short period of bottom resting is especially easy to misread when a pond has recently been cleaned or when the owner is observing outside the fish's usual active hours.

A koi that is probably compromised

A compromised koi may lie at an unusual angle, lean against a wall, roll, struggle to remain upright, or make repeated corrections with the fins. It may breathe quickly, gasp, hold the gill covers open, clamp the fins, flash, produce excess mucus, darken, isolate itself or fail to move when approached. It may rise briefly and then sink again because maintaining position is exhausting.

The change from that fish's normal behaviour matters more than a rigid rule such as “a koi should never sit on the bottom”. Compare the fish with itself, with the other koi, and with the same time of day on previous days.

The first question is whether one fish or the pond is affected

The number of affected koi is one of the most useful pieces of diagnostic information. It does not tell you the answer by itself, but it helps rank the possibilities.

The pond can still be part of the problem when only one fish shows signs. A robust koi may tolerate a marginal environment longer than a recently transported, injured or parasitised koi. Conversely, a single weak fish can be the first visible indicator of a problem that will affect the rest of the pond.

Low oxygen is the emergency cause to rule out first

Swimming, digestion, immune function and tissue repair all require oxygen. A koi with damaged gills or a high metabolic demand has even less reserve. When oxygen falls, fish may hover near returns, gather at waterfalls, breathe rapidly, become unwilling to swim and eventually settle on the bottom or gasp at the surface.

Oxygen problems are often missed because the pond looks clear and the air pump makes noise. The important question is whether water and air are actually being moved. A blocked diffuser, split airline, failing pump, clogged return, dirty filter, closed valve or power interruption can leave parts of a pond poorly oxygenated even while another area looks normal.

The risk is greatest after a hot day, during the hours before sunrise, after heavy feeding, when algae and plants respire at night, during a sudden algae die-off, or when organic debris is decomposing. Warm water holds less dissolved oxygen, while the fish, microbes and biofilter can consume oxygen faster. A heavily stocked pond can cross from apparently normal to dangerous more quickly than a lightly stocked pond.

If several fish suddenly become lethargic, breathe hard or gather at moving water, start backup aeration immediately. Do not wait for a test kit before responding to obvious respiratory distress. Create safe surface movement, check for blocked lines and inspect the pump. If you have a reliable dissolved oxygen meter, test at the time when the problem is worst and at more than one location. A surface reading at midday may not reveal a predawn crash or a stagnant bottom pocket.

For many freshwater fish, about 5 mg/L or higher is a useful practical target, but the safe margin depends on temperature, species, acclimation and existing gill damage. A fish already suffering from gill disease can be distressed at a reading tolerated by a healthy fish. The goal is a well-oxygenated system with reserve capacity, not a number that is checked only after the koi collapse.

Do not assume that an oxygen stone is enough. Aeration must match the pond's volume, depth, stocking density and weather, and it must remain available during a power failure. A second air pump on an independent circuit, a tested battery unit or a generator is valuable risk management for a valuable collection.

Water quality can make swimming exhausting

Koi experience the pond through their gills and skin every second. A pond may be visually attractive while ammonia, nitrite, chlorine, pH instability or a low oxygen reserve is causing physiological stress.

Ammonia

Ammonia comes from the koi's metabolism and the breakdown of food, faeces and decaying material. Test kits may report total ammonia nitrogen, or TAN, which includes ionised ammonium and the more toxic un-ionised form. The toxic fraction increases as pH and temperature rise, so a single TAN value cannot be interpreted without the surrounding conditions.

Ammonia irritates and damages the gills, interferes with the fish's internal ion balance and can cause quiet, abnormal swimming before obvious skin lesions appear. Koi may sit near returns, hold their fins close to the body, breathe faster or stop feeding. A pH rise in the afternoon can increase the risk even if the measured TAN has not changed.

Look for recent causes: a filter that was washed with chlorinated water, a new or immature biofilter, a hidden dead fish, a sudden increase in feeding, a blocked mechanical stage, an overcrowded tank, or a treatment that disrupted nitrifying bacteria. Correct the source, use properly treated replacement water, increase aeration and reduce or suspend feeding while the system stabilises.

Do not make a sudden pH correction simply because ammonia is present. A rapid pH rise can increase the toxic fraction of TAN. The response should be controlled and based on the measured problem rather than a panic reaction.

Nitrite

Nitrite is produced during the biological conversion of ammonia. It accumulates when the second stage of the nitrogen cycle is immature, damaged or overloaded. Nitrite can enter the bloodstream through the gills and interfere with the blood's ability to carry oxygen. Affected koi may act as if oxygen is low even when an oxygen meter gives a reasonable reading.

Test ammonia and nitrite as soon as lethargy appears and repeat the tests after corrective action. Also test the source water, especially if the pond has recently been topped up or changed. A normal reading after a large water change may not represent the earlier exposure.

Chlorine and chloramine

Untreated tap water can damage gill tissue and the protective mucus layer. Chloramine is more persistent and contains both chlorine and ammonia; removing only one part can leave another problem behind. Think carefully about the timing. Was a hose left running, was a filter cleaned beneath the tap, or was a conditioner dosed for the wrong water volume?

Test the source and pond water where possible, read the product label and calculate the actual volume rather than guessing. Clear water is not proof that it is safe water.

pH, KH and rain

Stable pH is generally more important than pursuing one “perfect” number. KH, or carbonate hardness, provides buffering against acidification. A low-KH pond can experience a sharp pH fall as nitrification and decomposition consume alkalinity. Heavy tropical rain can dilute minerals, carry leaves and soil into the pond, cool the surface layer and change the relationship between water layers.

A koi may become lethargic after a storm because several small stresses arrive together: temperature change, reduced oxygen, pH movement, runoff, suspended organic matter and sudden flow. Record pH and KH at consistent times and compare morning with afternoon when swings are suspected. Add buffers only after measuring the pond rather than treating the water by appearance.

Temperature and sudden change

Koi are ectothermic. Their muscle performance, digestion, oxygen demand and immune responses change with water temperature. In cooler water, activity and digestion usually slow. That can be normal if the fish remain balanced, responsive and otherwise healthy. A lethargic, clamped or abnormally breathing koi should not be dismissed as “just cold”.

Warm water increases metabolic demand while reducing the amount of oxygen the water can hold. It also increases the proportion of toxic un-ionised ammonia when pH is elevated. During a Malaysian heat wave, a koi may have more trouble swimming in the afternoon even though it looked normal in the morning. Shade and aeration are usually safer first responses than forcing more food or abruptly cooling the pond.

The change itself may be more stressful than the final temperature. A large water change, a rain-soaked tank, a pump failure followed by a fast refill, or moving a koi between systems with different chemistry can produce withdrawal and bottom sitting. Stabilise the pond, check replacement water, avoid additional handling and monitor the fish over the next several hours.

Do not deliberately raise the temperature to wake a lethargic fish or to test for koi herpesvirus. Temperature manipulation can accelerate some pathogens, increase metabolic demand and worsen oxygen problems. It is not a home diagnostic test.

Stress can look like laziness

Stress is a physiological condition in which the fish cannot maintain its normal internal state. It is not limited to a koi that visibly panics. The stress response redirects energy towards respiration, circulation and osmoregulation. Feeding, exploration and social activity become less important.

Common triggers include transport, a large catching session, spawning chases, aggression, overcrowding, a predator visiting, drilling or construction beside the pond, bright lights, a change in depth or flow, a filter clean, a salt change, and a new medication. Even a short event can leave a koi quiet for a while; repeated events can exhaust reserves and weaken the skin and mucus barrier.

A stressed koi may hide, darken, clamp its fins, remain at the bottom, breathe more quickly or hover beside the pond wall. It may resume normal swimming when the trigger is removed, but recovery does not prove that no disease is present. Stress makes parasites and bacteria more likely to become clinically important.

Reduce noise and handling, provide stable water and cover, avoid chasing the fish, and let the pond settle. If the behaviour is not improving or is accompanied by abnormal breathing, injuries or other signs, move from “stress” as an explanation to a structured diagnosis.

Gill and skin problems that make swimming difficult

The gills do more than collect oxygen. They remove carbon dioxide, maintain acid-base balance, excrete nitrogenous waste and help regulate ions. A koi with compromised gills may spend energy simply trying to breathe. It may not have enough reserve for normal cruising and may settle on the bottom to reduce the work of swimming.

Gill parasites and gill disease

Monogenean parasites, other external parasites, bacterial gill disease and chemical injury can cause rapid breathing, repeated yawning, flashing, excess mucus, clamped fins, separation from the group and a preference for well-moving water. Heavy gill infestation can make a fish less tolerant of low oxygen. Skin signs may be absent, and a skin scrape that does not include the gills cannot rule out a gill problem.

Fresh skin and gill examinations are most useful when taken from a live, clinically affected fish by someone trained to handle koi. One negative sample is not absolute proof that parasites are absent: sampling site, timing, previous treatment and examiner experience all affect the result.

Do not treat every bottom-sitting koi with a parasite medicine. Formalin and oxidising products can reduce oxygen and further irritate damaged gills. Praziquantel and other products target particular organisms, not every cause of lethargy. Use them only according to the exact label, the true water volume, the condition of the fish, aeration requirements and local rules.

Bacterial and systemic disease

A koi with bacterial disease may become lethargic with ulcers, fin erosion, red streaks, swollen areas, protruding eyes, abdominal enlargement, pale or damaged gills, abnormal faeces or severe weakness. These signs overlap with ammonia injury, nitrite stress, parasites and rough handling. Bacteria often take advantage of tissue damaged by one of those earlier problems.

Correcting water quality remains important, but it may not be enough after infection is established. Antibiotics do not treat poor water, parasites, viruses or stress. They can damage the biofilter and may fail when the fish is not eating medicated food. If bacterial disease is suspected, culture and susceptibility testing or advice from an aquatic veterinarian can make treatment more precise.

Viral disease and biosecurity

Viral disease in carp can begin with non-specific signs such as lethargy and reduced appetite. Koi herpesvirus disease may involve severe gill necrosis, respiratory distress, pale or mottled gills, sunken eyes, skin changes, neurologic signs and sudden mortality, but outbreaks do not always follow one exact sequence. Lethargy alone cannot diagnose KHV.

Concern is higher when several koi become ill over a short period, fish die unexpectedly, the problem follows new stock, or severe gill changes appear. Stop moving fish, nets, buckets and water between systems. Do not deliberately heat the pond to test for KHV. Contact an aquatic veterinarian or diagnostic laboratory and follow local movement and reporting requirements. A quarantine tank is useful not only for new arrivals but also for containing a suspected outbreak.

Buoyancy, injury and neurological problems

Bottom sitting is sometimes blamed on the swim bladder, but “swim bladder problem” is a description of a buoyancy symptom rather than a complete diagnosis. Koi have a two-chambered gas bladder that contributes to buoyancy. Buoyancy can be affected by inflammation, trauma, internal disease, pressure changes, abnormal gas, spinal injury, nutritional problems or damage to surrounding organs.

Observe the pattern carefully. A fish that sinks only when it stops swimming may be weak or unable to maintain buoyancy. A fish that floats, rolls, lists to one side, swims nose-up or nose-down, or cannot correct its position has a different problem from a fish that rests upright on the floor. Do not fast, feed peas, massage the abdomen or give medication as a universal response. Those home remedies may be inappropriate when the cause is gill disease, injury or internal infection.

Physical injury can also make a koi avoid swimming. A collision with the pond edge, a net, a predator or another koi may damage the spine, fins or body wall. Look for missing scales, blood, a bent posture, asymmetric movement, swelling or a fish that cannot turn normally. Avoid repeated catching. A fish-health professional can decide whether sedation and examination are safe.

Neurological disease is less common but more urgent when a koi spins, rolls, loses coordination, has tremors or repeatedly crashes into the pond wall. Toxins, severe oxygen deprivation, infection, trauma and internal disease can all be involved. Treat the event as more than simple tiredness and seek help promptly.

Feeding, constipation and bottom behaviour

A koi that is merely resting on the bottom may still rise for food. A koi that is lethargic after an unusually heavy meal may be uncomfortable, but appetite and posture should not be attributed automatically to “constipation”. Water temperature, food composition, gut disease and internal infection can all alter digestion. The appearance of faeces alone is not diagnostic because food colour, transit time and water temperature change it.

If the fish is alert and the water is sound, pause feeding briefly and remove any leftovers. Do not force food, give large amounts of soft food or add supplements to make the fish move. When feeding resumes, offer a small portion that the koi can finish and watch whether the fish can swallow normally. Persistent lethargy, weight loss, abnormal faeces, abdominal swelling or a sunken belly warrants a proper examination rather than repeated dietary experiments.

What to do in the first thirty minutes

When a koi suddenly becomes lethargic, use the following sequence.

1. Stop feeding temporarily

Remove floating food and do not keep dropping pellets to test the fish. Uneaten food decomposes, increases the nitrogen load and makes the pond harder to interpret.

2. Check air and water movement

Confirm that air pumps, diffusers, waterfalls, returns, skimmers and backup equipment are operating. If fish are breathing hard or several are affected, start additional aeration immediately. Create safe surface movement without transferring suspect water into another system.

3. Observe without catching

Count the affected fish and note their location: bottom, corners, shade, returns or areas of strong current. Look for rapid breathing, clamped fins, flashing, excess mucus, pale or ragged gills, ulcers, swelling, loss of balance, abnormal faeces and changes in colour. Write down what you see.

4. Test the water

Measure temperature, pH, KH, ammonia and nitrite. Test chlorine or chloramine after a water change. Measure dissolved oxygen if available, especially at the time the problem is worst. Take a source-water sample for comparison when the history points towards a water-change event.

5. Remove obvious hazards

Remove dead fish, rotting plants, leaves and uneaten food. Clean mechanical debris while preserving mature biological media. Do not strip a functioning biofilter in panic and create a new ammonia spike.

6. Reduce disturbance

Do not net the koi repeatedly for photographs. Handling removes mucus, increases oxygen demand and can spread contamination. If the fish is trapped, rolling or unable to maintain position, seek trained help instead of continuing the chase.

The first day: build a useful history

Create a timeline

Record the last time the koi looked normal, the first time it sat on the bottom, the number of affected fish, the weather, water changes, filter cleaning, power interruptions, new fish, transport, medication, feeding changes, spawning activity and any deaths. The timeline often reveals a cause that a single test does not.

The delay between an event and the behaviour matters. Chlorine exposure may follow a water change quickly. An oxygen crash may be worst before dawn. Parasite damage can become obvious after several days of stress. A bacterial or viral problem may develop over a longer interval. The most recent event is not automatically the cause.

Interpret results together

Ask whether the readings are stable for this pond and whether they explain the behaviour. A normal ammonia value after a large water change may not represent the original exposure. A normal daytime oxygen reading may miss a predawn crash. Record the test time, unit, method and whether the result was taken before or after a water change.

Decide whether to observe or isolate

If a single koi is mildly quiet but stable, moving it may add stress. If it is being bullied, cannot reach food, shows signs that could spread disease or needs close monitoring, an independent observation system may be appropriate. The system should have mature filtration or a controlled water-change plan, strong aeration, a secure cover and equipment that is not shared with the main pond.

Isolation is not a substitute for diagnosing the pond. A koi may improve in clean water because the original problem was environmental, while the main pond remains unsafe. A weak fish can also deteriorate quickly in a small, uncycled tank. Prepare the system before moving the fish.

Diagnosis before treatment

If the koi does not improve after oxygen, water quality and obvious stressors have been corrected, arrange a diagnostic examination. Depending on the history, a fish-health professional may recommend fresh skin and gill wet mounts, bacterial culture, blood work, radiography, necropsy or a targeted molecular test.

Samples are time-sensitive. Parasites can detach or die after the host dies, and fish tissue decomposes rapidly. A live, clinically affected fish often provides more information than a carcass that has been dead for several days. If a fish dies, ask the laboratory how it should be chilled and transported. Freezing can reduce the value of some examinations.

Bring water information with the koi. Record pond volume, temperature, pH, KH, ammonia, nitrite, recent treatments, feeding history, filter design, stocking density and recent additions. Good history narrows the possibilities before the first microscope slide is prepared.

Do not diagnose a parasite from a still photograph. Many gill organisms are invisible without magnification, and mucus or a reflective scale can be mistaken for a parasite. A negative sample also does not prove that parasites are absent. The sampling location, timing, previous treatment and experience of the examiner all matter.

Why random medication can make lethargy worse

A quiet koi naturally makes owners want to act. The problem is that the same visible sign can be caused by low oxygen, ammonia, injury, parasites, bacteria, viruses, temperature or stress. A treatment aimed at the wrong cause can add another chemical stressor.

Formalin and oxidising products can reduce oxygen and irritate compromised gills. Salt may help in selected nitrite or osmoregulatory situations, but it does not remove ammonia, chlorine, viruses or every parasite. Praziquantel targets certain flatworms; it does not correct water chemistry. Antibiotics do not treat parasites or viruses, and they should not replace culture or professional assessment when bacterial disease is suspected.

Medicated food has an additional limitation: a koi that is not eating may receive little or none of the intended dose. Bath treatments can damage the biofilter, and an inaccurate estimate of water volume can turn a label dose into an overdose. Do not mix products casually. Check the label, actual volume, fish condition, aeration requirements, filter compatibility, water-change instructions and local regulations.

The dependable order is to correct the environment, obtain a targeted diagnosis, and then choose a targeted treatment. In a genuine emergency, an aquatic veterinarian may recommend an initial treatment while tests are pending, but that decision should be based on the clinical picture and local professional advice.

Four practical examples

Example one: one koi sits on the bottom after transport

The other koi feed normally, but a new fish stays at the bottom, breathes a little faster and ignores food for three days. Transport stress is plausible, but it is not a reason to place an unquarantined fish directly into an established pond. Stop sharing equipment, review the quarantine decision, test the pond and, if safe, move the fish to a separate, strongly aerated observation system. A gill and skin examination may reveal parasites that cannot be seen from outside. Offer small amounts of food only while the fish stabilises.

Example two: the whole pond becomes quiet before sunrise

At 6:30 in the morning the koi are slow, several are close to the waterfall and one air diffuser is blocked. The pond is densely stocked, the previous afternoon was hot, and a large meal was given late in the day.

This pattern points first towards an oxygen problem and high organic demand. Increase aeration, clear the diffuser, stop feeding, inspect for decomposing material and test oxygen at the time of greatest risk. Once the pond is stable, reconsider whether the system has sufficient overnight reserve for the adult biomass.

Example three: a koi sinks and rolls after a collision

One fish rests on the bottom, leans to one side and makes repeated corrections. The water tests are normal, but a scale is missing near the flank and the fish cannot turn smoothly.

This is more consistent with injury, spinal or buoyancy involvement than with a simple feeding problem. Do not add a dewormer or force the fish to swim. Reduce disturbance, provide strong aeration and arrange a safe examination. Repeated netting could worsen the injury.

Example four: bottom sitting follows a storm and large water change

After heavy rain, the keeper replaces most of the pond water. The water looks clear, but the next day several koi sit low in the pond. KH is much lower than usual and pH changes greatly between morning and afternoon.

The immediate concern is instability rather than a mysterious disease. Test source and pond water, verify conditioner dose and true volume, restore buffering gradually according to measured results, keep aeration running and reduce feeding until the system is stable. If runoff may have entered the pond, professional testing may be warranted even when the water appears clean.

Malaysia and tropical-climate considerations

In Malaysia, koi ponds may remain biologically active throughout the year. There may be no long cool season that naturally reduces metabolic demand, and high temperature can coincide with heavy rain, power interruptions and rapid changes in water chemistry.

The most vulnerable periods are often the predawn hours after a hot day, the middle of a very hot afternoon, and the hours after a storm. Warm water supports active feeding and growth but also increases oxygen consumption, decomposition and filter load. Tropical rain can dilute KH, alter pH, bring organic material from roofs and hard surfaces, and cool only part of a pond.

Practical habits for a Malaysian pond include:

• Keep more than one reliable aeration route and test a backup pump, battery unit or generator before it is needed.
• Watch fish before sunrise after hot weather, heavy feeding, algae changes or a power interruption.
• Avoid making the largest meal during the hottest part of the day; split feeding into manageable portions.
• Use shade and circulation to reduce thermal stress without making sudden temperature changes.
• Test source water after municipal treatment changes, building work or plumbing maintenance.
• Treat every new koi as a quarantine risk, even when the seller is trusted. Use dedicated nets, bowls, hoses and towels.
• Keep short records of temperature, pH, KH, ammonia, nitrite, appetite and unusual behaviour.

Warm water does not make koi immune to disease. It can make a marginal system fail faster when oxygen and filtration capacity are limited.

Preventing lethargy before it starts

Prevention begins with knowing what normal looks like in your own pond. Observe the koi before feeding, during feeding and several minutes afterwards. Learn which fish are bold, which are cautious, which stay near the bottom, and how the group behaves at different times of day. A change is easier to see when you have a baseline.

Maintain stable water rather than repeatedly chasing extreme readings. Remove mechanical waste before it decomposes, but do not destroy mature biological media during cleaning. Match pump and aeration capacity to the adult biomass, not the size of the young koi currently in the pond. Keep emergency aeration ready and protect cables, air lines and pumps from accidental shutdown.

Feed what the fish can finish without creating a large surplus. Store food in a sealed, cool, dry container and replace it when the smell, texture or freshness changes. Introduce new foods gradually. If a timid koi is consistently excluded, improve distribution and assess the fish separately rather than assuming that more food will solve the issue.

Quarantine protects appetite as well as disease status. New koi can bring parasites, bacteria or viruses into a system, and the first shared sign may be that a resident fish becomes quiet or stops feeding. A properly prepared quarantine system uses dedicated equipment, reliable aeration and controlled observation. When KHV risk is relevant, professional guidance commonly includes at least 30 days of quarantine at approximately 24°C, but temperature manipulation should never be used as a do-it-yourself diagnostic experiment.

Practical checklist when a koi is lethargic

Record the following before making major changes:

• The time the koi was last seen normal.
• Whether one fish, several fish or the whole pond is affected.
• Whether the fish remains upright and responsive.
• Breathing rate, gill-cover movement and location in the pond.
• Temperature, pH, KH, ammonia, nitrite and dissolved oxygen when available.
• The time and result of each test, including whether it was before or after a water change.
• Recent weather, rain, water changes, filter cleaning, power interruption, construction or predator activity.
• New fish, transport, spawning, aggression, medication or salt changes.
• Food brand, batch, storage method, recent formulation change and uneaten food.
• Skin, gill, fin, faecal, swelling, wound, colour and buoyancy changes.
• Any dead fish, unusual smell, blocked filter or visible organic debris.

This record is far more useful to a fish-health professional than the sentence “my koi suddenly became lazy”.

Common mistakes

Assuming every bottom-sitting fish has a swim bladder disorder

Buoyancy is one possibility, not the default diagnosis. Oxygen, gill damage, ammonia, stress and systemic disease are more urgent to check.

Treating the visible symptom instead of the pond

A dewormer cannot repair chlorine damage, and an antibiotic cannot correct low oxygen. Test the environment before adding a product.

Waiting for all koi to become sick

One weak fish may be the first sign of a shared problem. Count and observe the entire group even when only one koi is sitting on the bottom.

Repeatedly catching the fish

Netting increases stress, removes mucus and can damage scales. Handle fish for a clear diagnostic purpose with clean, wet equipment and sufficient oxygen.

Making a large, untreated water change in panic

An unconditioned or temperature-mismatched change can add chlorine, chloramine, pH shock or thermal stress. Correct the measured problem with properly treated water.

Moving a sick koi into an uncycled container

A small tank can provide better observation, but it can also develop ammonia quickly. Prepare filtration, aeration, cover and a water-change plan before moving the fish.

When to seek urgent help

Contact an aquatic veterinarian, experienced fish-health professional or diagnostic laboratory promptly if:

• Several koi become lethargic within hours or days.
• Lethargy is accompanied by rapid breathing, surface gasping, loss of balance or severe weakness.
• Gills are very pale, dark, mottled, swollen or visibly damaged.
• There are ulcers, bleeding, severe fin erosion, swelling, protruding eyes or unexplained deaths.
• The problem follows a new koi, shared equipment or movement of water between systems.
• Aeration and water correction do not produce improvement.
• A koi spins, rolls, has tremors or cannot maintain position.
• KHV, another reportable disease or a major chemical contamination event is possible.

If a fish dies, preserve the timeline, water records and a representative fresh sample according to laboratory instructions. Do not sell, transport or give away koi that may have been exposed until the situation is understood.

Frequently asked questions

Is it normal for koi to sit on the bottom at night?

It can be normal for a koi to rest at night, especially when it remains upright, breathes normally and moves away smoothly when approached. Night-time rest becomes concerning when the fish is unresponsive, rolls, breathes rapidly, remains isolated, or looks abnormal during the day as well.

How long should I wait before treating a lethargic koi?

Do not use waiting as a substitute for checking the pond. Start aeration and test water immediately when the behaviour is new or severe. A calm, responsive fish with normal readings may be observed for a short period, but continued lethargy or any red-flag sign justifies diagnosis rather than passive waiting.

Should I add salt immediately?

Not automatically. Salt can be useful in selected nitrite or osmoregulatory situations, but it does not remove ammonia, chlorine, viruses or poor oxygen. Measure existing salinity, consider plants and other organisms, and seek professional advice when the fish is severely affected.

Can I feed peas or garlic to wake the fish up?

There is no universal food that treats lethargy. Peas, garlic and supplements can distract from gill, water-quality, infectious or injury problems. If the fish is alert and the environment is stable, offer a small amount of its normal, fresh food after the initial assessment. Do not force feeding.

Should I increase the water temperature?

Do not raise temperature simply to make the fish active or to test for KHV. Heating can increase oxygen demand, ammonia toxicity and pathogen activity. Correct the environment gradually and use professional guidance when a temperature-dependent disease is suspected.

Do I need to move the koi to a quarantine tank?

Only when the receiving system is already prepared with stable water, strong aeration, a cover and a filtration or water-change plan. Isolation is valuable for observation and biosecurity, but moving a very weak fish into an uncycled tank can be more dangerous than leaving it in a stable pond while you prepare.

If the water is clear, can the cause still be water quality?

Yes. Clarity does not measure oxygen, ammonia, nitrite, chlorine, chloramine, KH or pH stability. A clear pond can still be chemically stressful or poorly oxygenated.

Is lethargy always caused by disease?

No. Temperature, stress, social competition, a recent disturbance, injury and individual variation can all cause a short-term reduction in activity. The longer the change lasts and the more abnormal signs appear, the less reasonable it is to assume that the cause is harmless.

Conclusion

When a koi becomes lethargic or sits on the bottom, the most useful question is not “What medicine makes it swim?” It is “What changed, and what is the fish telling me about its environment or health?”

Check oxygen and circulation first, then test water, review recent events and observe the entire pond. Remove obvious waste, reduce disturbance and avoid stacking treatments without a diagnosis. If the fish is not improving, arrange a proper examination rather than guessing from posture or photographs.

Good koi keeping is not the ability to force a weak fish back into activity. It is the ability to recognise a meaningful change early, protect the pond while the cause is investigated, and choose treatment only when the evidence supports it.