Nitrite Poisoning (Brown Blood Disease): causes, symptoms and treatment

Nitrite Poisoning (Brown Blood Disease) (NO2- accumulation oxidizing hemoglobin to methemoglobin) — etiology, symptoms, diagnosis, active-substance medication, recovery and prevention; mortality without treatment: high.

Overview

Nitrite binds hemoglobin forming brown methemoglobin, blocking oxygen transport. Common during cycling between ammonia conversion and nitrite oxidation by Nitrospira. Causative agent: NO2- accumulation oxidizing hemoglobin to methemoglobin. Transmission: water. Incubation: 0-3 days. Reported mortality without treatment: high.

Symptoms

brownish gills (methemoglobin)
rapid gill movement
gasping at surface despite high O2
lethargy
loss of appetite
death in acute exposure

Causes

This is a water-chemistry condition, not an infection. Triggers include incomplete biological cycling, overfeeding, missed water changes, stocking density beyond the system's capacity, and equipment failures (heater spikes, CO2 solenoid issues, dead filter media). It is not transmitted between fish, but all stock sharing the affected water are exposed simultaneously.

Diagnosis

Diagnosis is based on water-test readings combined with clinical signs. Use liquid reagent test kits (more accurate than test strips) for ammonia, nitrite, nitrate, pH, and CO2/drop checker where applicable. Differentiate from infectious disease by the simultaneous deterioration of multiple species and the absence of skin/gill lesions other than respiratory distress.

Treatment

Treatment for a water-chemistry problem targets the root cause: restore safe parameters fast, protect gas exchange, and remove the toxic agent. No antibiotics or antiparasitics are appropriate — they will only stress the biofilter further.

Step 1: Quarantine

In a water-chemistry crisis there is no quarantine to set up — the entire affected system is treated as one unit. Do not move fish to another tank with mature biology, as you risk crashing that biofilter too. Add aeration, large water changes with dechlorinator, and a temporary product that detoxifies the relevant agent (chloride against nitrite, ammonia-binders against ammonia).

Step 2: Medication

Salt chloride competition + WC. Add aquarium salt 1 tsp/gal — chloride competes with nitrite at gill uptake. Large water change with dechlorinator. Prime to bind nitrite for 48h. (duration: until nitrite 0)

Step 3: Recovery

Recovery means restoring the cycle: re-test ammonia, nitrite, nitrate, pH, and salinity (or specific gravity) daily until readings stabilise within target ranges for the stocked species. Reseed the biofilter from a mature source if needed, reduce feeding while the cycle catches up, and add no new fish until parameters are stable for at least two consecutive weeks.

Prevention

fully cycle tank
test nitrite weekly during cycling
seed beneficial bacteria
avoid antibiotics that crash cycle