Nitrite is the intermediate product in the nitrogen cycle, produced when beneficial bacteria convert ammonia but before a second group of bacteria converts it to relatively harmless nitrate. When the biological filter is overwhelmed, disrupted, or immature, nitrite accumulates. It enters the bloodstream through the gills and binds to hemoglobin, forming methemoglobin, which cannot carry oxygen. This condition is sometimes called brown blood disease because affected fish literally have chocolate-brown blood.

Common causes

New tanks that have not completed cycling are the primary culprit. Overfeeding, sudden increases in stocking density, filter cleanings done with chlorinated tap water that kills beneficial bacteria, antibiotic treatments, and power outages that starve filter media of oxygen all disrupt the nitrogen cycle and can cause nitrite to spike.

Symptoms

Fish gasp at the surface despite adequate aeration. Gills may appear darker than normal. Lethargy, loss of appetite, clamped fins, and erratic swimming are common. In severe cases, fish lie on the bottom, breathing rapidly, and mortality can be sudden. Bottom-dwelling species and scaleless fish tend to be affected earliest.

Emergency response

Perform an immediate large water change of 50 percent or more, matching temperature and treating replacement water with dechlorinator. Add aquarium salt (sodium chloride, not marine salt) at a rate of one to three grams per liter; chloride ions compete with nitrite for absorption at the gill, providing a degree of protection. Reduce feeding to a minimum and test water parameters every few hours.

Longer-term stabilization

Seed the filter with mature media from an established tank if available. Add a quality bacterial supplement to accelerate cycle recovery. Avoid cleaning the filter during the recovery period. Monitor nitrite and ammonia daily until both read zero for at least a week. Gradually resume normal feeding only after the cycle is stable.

Prevention

Cycle new tanks fully before adding fish. Avoid overstocking. Clean filter media in tank water rather than tap water. Maintain a consistent feeding routine and remove uneaten food promptly. Keep a liquid test kit on hand; test strips often lack the precision needed to catch early nitrite rises.

Veterinary disclaimer: This article is for general education only. For fish health emergencies, consult an aquatic veterinarian or experienced aquaculture specialist when available.