Koi Health: Signs of Illness, First Checks and Getting Help

Hanna HI761 measures total chlorine in its specified range. It is a separate test from ammonia, nitrite or pH.

When a koi looks unwell, check the pond as well as the fish

A fish that is gasping, lying apart or refusing food needs a closer look. Start by watching the whole pond and checking the water and equipment. The same visible sign can have several causes, and adding a treatment before those checks can make the problem harder to understand.

If fish are in obvious distress, losing balance or dying, act promptly and get appropriate help. Support aeration where it is safe to do so, stop adding food while the immediate problem is assessed and check whether circulation has failed. Do not spend hours trying to identify a disease from pictures while the fish remain in poor water.

This guide helps you gather useful information and decide what needs attention next. It covers first checks, common signs and when a fish vet or the Fish Health Inspectorate should be involved. Our separate everyday koi care guide covers the routine between problems.

First checks when fish are distressed

1. Look at the group. Is one fish affected, or several? Are they gasping, gathering at an inlet, rubbing, losing balance or simply less active?
2. Check circulation and air. Look at actual water movement and the diffusers, not just whether a pump is making a noise.
3. Measure the water. Record temperature, ammonia, nitrite and pH; check KH and other relevant parameters. Measure dissolved oxygen if suitable equipment is available.
4. Check what changed. Include new fish, treatment, feeding, filter cleaning, top-up, rain, building work and power interruptions.
5. Get help with the results. Severe distress, wounds, losses or a problem that is not resolving need appropriate professional assessment.

Do not move a distressed fish into an unfiltered bucket simply to isolate it. If separate accommodation is needed, it must provide suitable conditions. Handling and transfer can add stress, so agree the purpose and method before catching the fish.

Write down results before changing things

A useful record includes the actual number, units, test method and time. “Water is fine” is not enough to compare with a later result or to help someone assess what happened.

Where practical, take a water sample and record conditions before a major change, without delaying urgent support for the fish. Ask the professional advising you how to collect and store any sample they need; different tests and investigations have different requirements.

Keep photographs of product labels and a list of anything added to the pond. Include the amount, time, estimated or measured pond volume and whether the filter or UVC was changed. A product described as natural is still relevant to the history.

Water problems can look like illness

Ammonia, nitrite, unsuitable pH, disinfectant in incoming water and low oxygen can all affect fish. They may also make fish more vulnerable to other problems. Checking them first is not dismissing the possibility of disease; it establishes the conditions the fish are living in.

Ammonia readings need the test’s reporting basis and the pH and temperature for interpretation. Nitrite is a different measurement with a different effect. A zero ammonia result does not rule out nitrite, and clear water rules out neither.

Our ammonia and nitrite guide explains these differences, including the units used by NT Labs and Hanna tests. Our pH, KH and GH guide covers buffering and changing pH.

If a problem followed a refill, include the source water and dechlorination arrangement in the checks. Chlorine and chloramine cannot be judged by whether the water looks or smells normal. Use an appropriate test and seek advice about the incoming supply and treatment.

The manufacturer’s HI761 information explains that model’s measurement and range. Match the test to the question; a free-chlorine result is not automatically a total-chlorine result, and a zero on one scale is not proof that every water hazard is absent.

Gasping, rapid breathing or gathering near a return

Check aeration and water movement immediately. Look for a stopped pump, blocked intake, disconnected airline or a change after filter maintenance. Consider the weather, feeding and organic load.

Keep the aeration suitable for the pond while obtaining help, but continue the checks. Fish with nitrite exposure or gill damage may show breathing difficulty even where the water contains oxygen. Keep checking the water even when the air pump is running.

Measure dissolved oxygen with a suitable method where possible, and record the time and temperature. Afternoon and early-morning conditions can differ. Do not use a universal litres-of-air-per-litre-of-pond ratio as proof of adequate oxygen.

Our aeration guide explains air pumps, stones and membrane diffusers. For a sudden problem in a large or deep pond, avoid an unplanned attempt to mix stagnant bottom water through the whole pond; get site-specific advice.

Rubbing, flashing or jumping

A koi turning briefly to rub against a surface is often called flashing. Persistent or repeated rubbing needs investigating, but it does not identify a particular parasite.

Check water conditions and recent changes first. Record whether one fish or several are affected and whether there are other signs such as breathing changes, excess mucus, clamped fins or visible damage.

If examination for parasites is indicated, have it carried out by someone competent to sample and interpret the findings. A microscope result needs the fish’s condition and water history alongside it. Do not keep changing treatments because rubbing continues after an unconfirmed guess.

Not eating, hiding or staying apart

Appetite and activity change with temperature, disturbance and conditions. A new fish may be cautious, and a fish that normally keeps away from people may not join a hand feed. The question is whether its behaviour has changed and whether other concerns are present.

Look at breathing, balance, body condition and the rest of the group. Check water and equipment. If the fish remains apart, deteriorates or has visible abnormalities, seek assessment rather than assuming it is simply shy.

Do not add extra food that remains uneaten. It creates waste and does not help establish the cause. Our shy koi guide covers calm observation and feeding habits; persistent illness signs belong in a health investigation.

White spots, fuzzy areas, excess mucus or changing colour

Describe what you can see without immediately naming it. Is it a raised spot, a flat patch, a wound edge, an area of mucus or material attached to the surface? When did it appear, and is it changing?

Several conditions can look similar in photographs, especially through moving water. Lighting and reflections can also exaggerate a mark. A clear image is useful evidence, but it is not a substitute for appropriate examination.

A fuzzy growth on damaged skin does not by itself establish the original cause of the damage. Treating only what is visible can leave a water problem, injury or underlying infection unresolved.

Sores, damaged fins and injuries

Seek prompt advice for open sores, significant wounds or deterioration. Keep the water conditions and equipment under review while arranging assessment.

Look for possible hazards such as sharp fittings, damaged nets, predator access or a recent handling event. Record them as possibilities, not conclusions. An injury can become complicated by infection, and a sore should not automatically be labelled a simple scrape.

Do not apply household antiseptics, human medicines or a mixture of pond products to a fish. Sedation, injections and other invasive procedures need suitable professional direction. A fish vet can advise on an appropriate treatment after assessing the fish and the cause.

Swelling, raised scales or loss of balance

These signs need assessment. Terms such as dropsy or swim-bladder problem can describe an appearance without establishing the underlying cause.

Record the development, whether the change is general or local, appetite, breathing and any recent events. Avoid repeatedly catching or turning the fish to obtain a perfect photograph.

A fish unable to maintain normal position or deteriorating rapidly needs prompt help. Improving the water is important, but it is not a reason to postpone assessment where the fish is seriously unwell.

One fish affected or the whole pond?

A whole-group change makes shared conditions particularly important: oxygen, incoming water, a treatment, power loss or another exposure. One affected fish can still be the first sign of a wider issue, so check the pond rather than assuming it is isolated.

The timeline helps. Did the change follow a top-up, storm, filter clean or new fish? Did signs begin suddenly or develop over days? Have neighbouring activities introduced a possible contaminant?

Avoid changing several things at once without a reason. Where urgent intervention is necessary, record what was done and when. That makes the response easier to assess and prevents important details being lost.

When to involve a fish vet

Contact a vet with fish experience for serious or persistent illness, significant injuries, unexplained losses or when diagnosis and treatment are uncertain. Ask what information and samples they need before transporting a fish.

A pond specialist can help investigate the water, filtration and equipment. For clinical tests or prescription treatment, you need a fish vet. Give them your water results and the history of the problem so they have the full picture.

Merck’s fish disease guidance explains the overlap between environmental and infectious problems. Its wider veterinary material is not an invitation to transfer drug doses or procedures into a home pond without proper direction.

KHV and other listed disease concerns

In England and Wales, suspected listed fish disease must be reported immediately to the Fish Health Inspectorate. The official advice also requires reporting known or suspected increased mortality. Do not wait for an online group to agree on a diagnosis.

Keep fish in place and prevent potentially contaminated water or equipment being moved while obtaining instructions. Follow any controls and sampling advice given. The current official reporting page includes the contact route and a 24-hour on-call service for urgent reports.

KHV guidance explains that infected fish can spread the virus without looking sick. Do not describe a pond as free of disease just because the remaining fish are swimming normally.

Using a treatment after the cause has been assessed

Confirm the intended problem, exact product, correct pond volume and instructions before treatment. Account for fish species, temperature, water conditions, aeration, existing salt or other treatments, and any filter or UVC instructions.

Keep the current label and record the dose and time. Similar product names, older formulations and internet advice may not match the pack you have. Do not combine products or increase a dose to make it work faster.

Ask how success will be assessed and what follow-up is needed. Fish appearing brighter for a short time is useful to note, but it may not show that the original cause has been resolved. Continued deterioration requires further advice.

Useful equipment to keep ready

Keep in-date water tests, a thermometer, equipment records and suitable contact details accessible. Have a way to support aeration during a failure and know how it is used safely.

Suitable handling equipment can be useful, but it needs to match the fish and the task. A bowl does not provide filtration or aeration by itself. Keep quarantine tools separate and avoid moving equipment between ponds without an appropriate cleaning and disinfection plan.

For regular checks, the NT Labs multi-test kit and individual Hanna ammonia and nitrite Checkers are options to compare. The latter models report nitrogen-based units; our water testing guide explains the distinction.

Questions about koi health

Should I treat the whole pond if one koi rubs?

Not on that sign alone. Check water, observe the group and obtain an appropriate examination if the behaviour persists or other signs appear. Treatment should follow an assessed cause.

My water tests are normal. Can it still be a water problem?

Check what was measured, when and with which method. A basic kit may not cover oxygen or chlorine, and a reading after the event may differ from conditions during it. Give the actual results to the person advising you.

Does a large air pump rule out oxygen trouble?

No. Air delivery depends on the installation, while oxygen demand depends on conditions. Check the system and measure dissolved oxygen where indicated; continue investigating other causes of breathing difficulty.

How long can a koi go without eating?

There is no single useful deadline for an unwell fish. Temperature, condition and the reason for not eating matter. Do not use a number of days as permission to leave a deteriorating fish unchecked.

Should I move an ill fish to a friend’s pond?

No. It can spread a problem and exposes the fish to another transfer and different conditions. Seek advice about suitable separate care where needed, and follow official instructions if disease controls may apply.

What should I send when asking for help?

Send the water results with units, temperature, pond volume, equipment details, timeline and a short video. Include new fish, treatments and losses. Clear information helps the next step far more than a photograph labelled with a guessed disease name.