Nitrites in the pond: emergency steps to save your fish, then how to lower the level sustainably

Are your fish staying on the bottom, breathing rapidly near the surface, or refusing food even though the weather is fine? Before blaming a parasite, test the nitrites. Nitrites are among the most frequent and most misidentified causes of death in garden ponds, and a high level can kill within a few days. The good news: the emergency measures are simple, and the cause is almost always identifiable.

Why are nitrites so dangerous?

Nitrites (NO₂⁻) are an intermediate step in the nitrogen cycle. Fish waste and leftover food release ammonia, which bacteria attached to the filter oxidize into nitrites, which other bacteria then convert into nitrates, which are much less toxic. When the second step does not keep up, nitrites accumulate.

Absorbed through the gills, they oxidize the blood's hemoglobin into methemoglobin, which cannot transport oxygen. A fish poisoned by nitrites suffocates in water that is nevertheless well oxygenated: its blood can no longer bind oxygen. In severe cases, the gills take on a brownish tint, hence the name "brown blood disease" used in fish farming. The resulting chronic stress also weakens defenses and paves the way for parasites and infections.

Thresholds to know

Actual toxicity varies depending on the species, temperature, oxygen levels, and especially the chloride content of the water. The same level will be better tolerated in water rich in chlorides than in very soft rainwater.

Signs that should raise concern

• fish that remain motionless at the bottom or, conversely, gasp at the surface;
• rapid breathing, with fast-moving gill covers;
• reduced appetite or refusal to eat;
• fish rubbing themselves or with clamped fins, often because an opportunistic infection is taking hold;
• gradual mortality, especially among the larger fish.

Emergency actions, in order

1. Stop feeding for two to three days. Less food means less ammonia and therefore fewer nitrites. An adult fish can easily tolerate a few days of fasting.
2. Change 20 to 30% of the water with dechlorinated water at a temperature close to that of the pond. Repeat every two days if necessary. Do not perform a massive water change, which would stress the fish further.
3. Aerate as much as possible: air pump, water jet, waterfall. A fish whose blood transports oxygen poorly needs water that is as oxygenated as possible, and so do nitrifying bacteria.
4. Protect with non-iodized pond salt. Chloride competes with nitrites at the gills and reduces their absorption: this is a classic aquaculture technique. A low dose, around 1 kg per m³, is commonly used for protection. Calculate the volume precisely, dissolve the salt before adding it, and remember that it does not evaporate: it only disappears with water changes. Avoid high doses if the pond is heavily planted.
5. Seed the filter with live nitrifying bacteria, directly in the filter, with the pump running.
6. Measure KH. Nitrifying bacteria consume alkalinity; below approximately 5 °dH, their activity slows and the pH can become unstable. Correct it if necessary.

Why does my pond have nitrites?

A filter that is too new

A new filter takes several weeks to develop its beneficial bacteria. The bacteria that convert nitrites establish themselves more slowly than those that produce nitrites: this is the classic spike in new ponds, often occurring a few weeks after filling.

A filter washed with tap water

Chlorine in tap water destroys much of the filter's beneficial bacteria. Always rinse the filter media in a bucket of pond water, and never clean all compartments on the same day.

A stopped pump

A few hours without circulation are enough to deprive the bacteria of oxygen. When restarted, the filter also releases water containing a high level of waste.

Too many fish or too much food

Every pellet eventually becomes ammonia. An overstocked pond or generous feeding quickly exceeds the filter's capacity, especially as the fish grow.

Spring and cold water

Nitrifying bacteria are greatly slowed down in cold water. If you feed too early in spring, before the filter has restarted, nitrite levels rise.

Medication treatment

Some antiparasitic or antibacterial treatments also affect the filter's beneficial bacteria. Monitor nitrites after each treatment.

Prevent nitrite spikes

• Size the filter for the volume and fish load, not just the volume.
• Feed according to the water temperature, in quantities that are consumed within a few minutes.
• Let the pump run continuously.
• Maintain a sufficient and stable KH.
• Maintain the bacterial flora after every cleaning and restart.
• Test nitrites in spring, after adding fish and after any incident.

Also read

To avoid being caught off guard: How to perform a water analysis of my pond or pool?

READ THE ARTICLE

Frequently asked questions

— What nitrite level is dangerous for fish?

Ideally, the goal is zero. Above 0.3 mg/L, fish are stressed, and above 0.5 mg/L, the situation becomes dangerous.

— How long does it take to lower nitrites?

With fasting, water changes and bacteria, the level often decreases within a few days. A lasting return to zero depends on the filter recovering.

— Is salt useful against nitrites?

Yes: chloride reduces nitrite absorption through the gills. Use a low dose of non-iodized salt, carefully calculating the volume.

— Should the filter be cleaned in case of nitrites?

No, definitely not thoroughly. You would remove the bacteria the pond needs. Simply rinse it in pond water if the flow is too low.

In summary

• Nitrites prevent fish blood from carrying oxygen.
• Above 0.3 mg/L: fasting, partial water change, aeration, salt, bacteria.
• Check the KH, which is essential for nitrifying bacteria.
• Common causes: young or poorly cleaned filter, pump stopped, overload.
• Test every day until the level returns to zero.

Last updated: September 28, 2026.