Dead fish in the pond: the 9 causes to check (and how to find the right one in 30 minutes)

Discovering a dead fish in your pond is always a shock. Discovering several is an emergency. The usual reaction is to buy a treatment “just in case.” This is often a mistake: the vast majority of deaths in ponds are not caused by a disease, but by the water itself. And adding medication to water that is already unbalanced can make the situation worse. Here is the method we use at our garden center to find the cause, in the order it should be investigated.

The three questions to ask yourself first

1. Which fish are dying? All the fish at the same time, regardless of size, points to the water. The largest fish dying first points to oxygen. One individual dying from time to time points to disease or old age.
2. When do you find them? Early in the morning, in summer: oxygen. A few days after adding water, cleaning the filter, or introducing new fish: water quality or contagion.
3. How quickly? Mass deaths overnight are almost always caused by a water or oxygen problem. Regular deaths over several weeks are more suggestive of a parasite or infection.

Also examine a dead fish closely: gills, fins, and skin. Very pale or brown gills, spots, ulcers, or eroded fins are all clues.

The 9 causes, from most frequent to least frequent

— 1. Lack of oxygen

The leading cause in summer. Oxygen levels drop throughout the night and reach their lowest point at dawn, especially when the water is warm, green, or laden with organic matter. Signs: deaths discovered in the morning, larger fish affected first, fish at the surface or beneath the waterfall. Solution: aeration running at night, pump never switched off.

— 2. Ammonia and nitrites

Produced by waste and food, they are normally converted by filter bacteria. They accumulate in a new pond, after overly thorough filter cleaning, after a pump stops, in cases of overfeeding or overcrowding. Signs: lethargic fish, brown gills, rapid breathing, sometimes very clear water. Above 0.3 mg/l of nitrite, action is required.

— 3. A pH crash

In water low in lime (low KH), typical of ponds filled with rainwater or located in granite regions, the pH can collapse suddenly after heavy rain or a CO₂-rich night. Signs: KH below 4 °dH, pH below 6.5, sudden deaths of all the fish.

— 4. Chlorine in the mains water

A major refill or top-up with tap water without a water conditioner may be enough. Chlorine attacks the gills and also kills the filter bacteria, which then causes a nitrite spike. Sign: deaths within 24 to 48 hours after adding water.

— 5. Parasites and infections

White spot, costia, trichodina, flukes, bacterial infections (Aeromonas), and, in koi, the dreaded herpesvirus (KHV). They strike mainly in spring, when the water warms up and the fish's immune system is still sluggish, or after a new fish is introduced without quarantine. Signs: fish rubbing themselves, clamped fins, spots, ulcers, deaths spread over time.

— 6. Predators

Herons, cats, kingfishers, otters, or raccoons. They rarely kill without taking their prey away: instead, fish tend to disappear, or fish are found with injuries to their sides and die a few days later from an infection.

— 7. Pollution from outside

Pesticides or fertilisers running off from the garden, wood treatment from a deck or jetty, fresh unneutralised cement, copper or zinc items, barbecue smoke blown onto the water. Signs: sudden deaths after nearby work or treatment, while the usual water tests are normal.

— 8. Thermal shock and winter

A major cold water change in summer, or a pond completely covered by ice for several days: gases from decomposition cannot escape. Signs: fish dead when the ice thaws.

— 9. Overpopulation and chronic stress

It is not a cause in itself, but it amplifies all the others. An overstocked pond is constantly on the verge of collapse: the slightest heatwave or the smallest oversight is enough to tip it over.

Emergency steps to save the survivors

1. Remove dead fish immediately. Their decomposition pollutes the water and can spread an infection.
2. Provide as much oxygen as possible: air pump, waterfall, water jet.
3. Stop feeding for 48 hours.
4. Test the water: ammonia, nitrites, pH, KH and temperature.
5. Perform a partial water change of 20 to 30% using water at a similar temperature and treated to remove chlorine, if the tests reveal a problem.
6. Do not add any medication until the water is healthy and the cause has been identified.

If nitrites or ammonia are involved, a neutraliser works within a few hours, while adding nitrifying bacteria restarts the filter so the problem does not return the following week.

Read also

How can you prevent fish deaths in the pond?

READ THE ARTICLE

Frequently asked questions

— My fish died even though the water is perfectly clear. Why?

Clear water may be lacking oxygen or contain nitrites, invisible to the eye. This is actually common after thoroughly cleaning the filter. Only a test can determine the cause.

— Should the pond be drained after fish deaths?

Almost never. A complete drain destroys the biological balance and stresses the survivors. A partial water change and correcting the cause are sufficient in the vast majority of cases.

— My two goldfish died despite good oxygenation. What should I check?

In this order: nitrites, pH and KH, chlorine if you have added tap water, then the condition of the gills and skin to look for a parasite. Also consider any recent treatment or work around the pond.

— When can new fish be added?

Once the cause has been identified and corrected, and after at least two weeks of stable parameters. If a disease is involved, wait until the survivors have fully recovered and quarantine newcomers.

In summary

• Most fish deaths are caused by water quality, not disease.
• Who dies, when, and how quickly: three decisive clues.
• Test the water before treating it.
• Emergency: remove dead fish, oxygenate the water, fast the fish, and perform a partial water change.

Last updated: September 28, 2026.