What is Nitrite poisoning in Koi?

Nitrite poisoning, often called “Brown Blood Disease” is a dangerous water quality condition that affects koi when nitrite levels rise in the pond.

Nitrite (NO₂⁻) enters the bloodstream through the gills and interferes with oxygen transport, effectively suffocating the fish even when oxygen levels in the water are normal.

The term “Brown Blood Disease” comes from the fact that affected koi develop chocolate brown colored blood instead of healthy bright red blood.

What Causes Nitrite Poisoning?

Nitrite is part of the nitrogen cycle:

Ammonia converts to nitrite which then converts to nitrate

Problems occur when:

• The biological filter is immature or damaged

• Beneficial bacteria are insufficient

• The pond is overloaded with fish or waste

Common Causes

• New pond or newly cycled filter

• Overfeeding

• Overstocking

• Filter crash (after cleaning or medication)

• Dead organic matter (leaves, uneaten food, dead fish)

• Sudden increase in ammonia levels

Why Nitrite is so Dangerous

Nitrite enters the bloodstream through the gills and converts hemoglobin, which:

• Cannot carry oxygen

• Causes internal suffocation

• Leads to rapid stress and death if untreated

Even low levels (0.25 ppm) can begin causing stress.

Symptoms of Nitrite Poisoning

Nitrite poisoning often looks like an oxygen issue, which makes it easy to misdiagnose.

Early Signs

• Lethargy

• Reduced appetite

• Hanging near water returns or aeration

• Clamped fins

Advanced Symptoms 

• Gasping at the surface

• Rapid gill movement

• Brown or pale gills

• Fish grouping near inflow areas

• Weak swimming or isolation

Severe Cases

• Loss of balance

• Collapse

• Sudden death

How to diagnose It

Test Your Water

Use a reliable test kit to check:

• Nitrite

• Ammonia

• Nitrate

Any detectable nitrite is a problem

Safe level: 0 ppm

Rule Out Other Causes

Nitrite poisoning is often confused with:

• Low oxygen

• Parasites affecting gills

• Ammonia poisoning

If oxygen is adequate and fish are still gasping, suspect nitrite.

Emergency Treatment (Act Fast)

Nitrite poisoning is treatable, but timing matters.

Add Salt (Immediate Protection)

Salt (sodium chloride) prevents nitrite from entering the bloodstream.

Dosage:

• 0.1%-0.15% salt concentration

• (1-1.5 pounds per 100 gallons)

Add gradually over several hours.

Partial Water Change

• Change 25-50% of pond water

• Use dechlorinator if needed

• Match temperature to avoid shock

Increase Aeration

Even though nitrite affects blood oxygen transport, boosting oxygen helps reduce stress.

• Add air stones 

• Increase waterfall flow 

• Use additional pumps if available

Stop Feeding

Do not feed until nitrite returns to 0.

Feeding increases ammonia, which becomes more nitrite.

Long Term Fix (Root Cause)

Emergency treatment helps fish survive, but you must fix the underlying issue.

Improve Biological Filtration

• Ensure filter is properly sized

• Add bio-media if needed

• Avoid over-cleaning filters

• Increase KH levels

Support Beneficial Bacteria

• Maintain stable water conditions

• Avoid sudden chemical changes

Reduce Bioload

• Feed less

• Remove debris

• Avoid overstocking

Monitor Water Daily

Until stable:

• Test nitrite every day

• Continue partial water changes as needed

Prevention

Nitrite poisoning is almost always preventable.

Key Prevention Steps

• Maintain a mature biofilter

• Test water regularly

• Avoid overfeeding

• Clean pond responsibly (don’t wipe out bacteria)

• Quarantine new fish

• Remove organic waste promptly

When It’s an Emergency 

Treat immediately if you see:

• Fish gasping at the surface

• Multiple fish affected at once

• Nitrite reading above 0.25 ppm

• Sudden behavior changes across the pond

This is a pond-wide issue, not an individual fish illness.

Final Thoughts

Nitrite poisoning is one of the most common, and misunderstood, koi health issues.

The key takeaway:

If your koi are gasping but oxygen seems fine, test for nitrite immediately.

With fast action and proper pond management, it’s completely preventable, and reversible.

Frequently Asked Questions

How fast can nitrite kill koi?

Severe spikes can cause death within 24-72 hours if untreated.

Can koi recover from nitrite poisoning?

Yes, if treated early with salt and water changes.

Is salt always necessary?

In emergencies, yes, it’s the fastest way to block nitrite toxicity.

What’s the difference between ammonia and nitrite poisoning?

• Ammonia burns gills and skin

• Nitrite prevents oxygen transport in blood

Both are dangerous, but nitrite often causes hidden suffocation.