Goldfish dying despite good oxygenation: causes to check

“My two goldfish died, yet the pond is well oxygenated. Why?” We receive this message in dozens of forms every year. And almost every time, the person is right about one thing: oxygen is not the culprit. The problem is that a pond can have lovely bubbles on the surface and toxic water below. When oxygen is ruled out, the usual suspects are ammonia and nitrites, pH, tap water added without precautions, a toxin from outside, an introduced disease, or thermal shock. Here is how to investigate.

First, are you sure about the oxygen?

One note before ruling out this possibility. From the bank, you see bubbles and movement on the surface. What the fish experience is the oxygen level at the bottom at 5 a.m. Boyd and Tucker explain it very well: in a pond, oxygen reaches its lowest level just before dawn, after a night during which plants, algae, fish, and sediment have all respired. If your fish were found dead early in the morning after a hot, stormy night, keep oxygen on the list of suspects, even with a pump running.

The investigation table

Ammonia and nitrites: the invisible killers

Fish release ammonia continuously through their gills. In a balanced pond, filter bacteria convert it into nitrites, then into much less toxic nitrates. When this system breaks down, the water remains clear, bubbles continue to rise, and yet it becomes toxic. In a healthy pond, ammonia and nitrites should be undetectable with a standard pond test: any measurable value is a warning.

The risk situations are well known: a new pond whose filter has not yet been colonized, a filter rinsed with tap water, too much food in hot weather, or adding several fish at once. Noga points out that exposure to nitrites interferes with oxygen transport in the blood: a fish can therefore suffocate in water that is otherwise well oxygenated. This is very likely the answer to many questions like yours.

pH, KH, and tap water

KH, or carbonate hardness, acts as a buffer. In water with low KH, often rainwater or a heavily planted pond, the pH can drop sharply, sometimes overnight. Conversely, a high pH makes ammonia much more toxic.

Tap water, on the other hand, is treated to make it safe to drink, in accordance with European Directive 2020/2184, which often involves disinfection. What is good for us is not necessarily good for gills. A small top-up to compensate for evaporation rarely causes a problem; a large refill all at once, without a water conditioner and with much colder water, is a classic cause of death within 48 hours.

Toxins, diseases, and shocks

If the water is perfect, look around the pond. A moss treatment on a roof whose rainwater ends up in the pond, weed killer sprayed by the wind, treated wood around the edge: sometimes, small amounts are enough. Also think about fish added recently. A quarantine period of a few weeks, a basic aquaculture practice according to Noga, prevents parasites and bacteria from being introduced into an established pond. Finally, goldfish are hardy but not indestructible: a small, shallow pond that heats up strongly in the afternoon and cools down at night will exhaust them.

What should you do immediately for the survivors?

Remove the dead fish. Test the water. If there is ammonia or nitrites, perform a partial water change of about a quarter, using conditioned water at a similar temperature, and repeat if necessary. Stop feeding for two or three days. Do not start any medication without identifying the cause: adding another product to water that is already degraded often makes things worse.

Also read

To prevent this from happening again: How to prevent fish deaths in the pond?.

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Frequently asked questions

— Can a fish die of asphyxiation in well-oxygenated water?

Yes. Nitrites interfere with oxygen transport in the blood, and gills damaged by ammonia or parasites absorb it poorly. The fish then suffocates in water rich in oxygen.

— Should you empty the pond after fish have died?

No. Completely emptying the pond destroys beneficial bacteria and stresses the survivors. Partial water changes are sufficient in the vast majority of cases.

— How long should you wait before adding fish again?

Until the cause has been identified and the tests remain good for several weeks. Then introduce new fish gradually.

— Do goldfish die of old age in a pond?

They can live for many years in a pond. Two deaths close together point more to an environmental cause than to age.

In summary

• Bubbles on the surface do not guarantee oxygen at the bottom at dawn.
• Ammonia and nitrites are the first suspects: they must be undetectable.
• Check pH and KH, and beware of large additions of tap water.
• Look for a recent event: new fish, treatment, storm, heat.
• Perform partial water changes and stop feeding before any treatment.

Last updated: September 29, 2026.