Nitrite

Nitrogen compound (NO₂⁻) produced when ammonia-eating bacteria break down ammonia. Toxic to fish; converted to less-toxic nitrate by a second bacterial group. In a cycled tank, nitrite should read 0.

Nitrite is the middle stage of the nitrogen cycle. It's produced from ammonia by Nitrosomonas bacteria and consumed by Nitrobacter / Nitrospira bacteria. During fishless cycling, nitrite typically appears 1-2 weeks after ammonia and lingers until the nitrite-eating bacteria catch up.

Nitrite poisoning damages fish gills and impairs oxygen transport (it binds hemoglobin, causing 'brown blood disease'). Symptoms: gasping at the surface, lethargy, rapid gill movement.

Adding chloride (e.g. via a small dose of plain aquarium salt or sodium chloride at 1 tsp per 5 gallons) blocks nitrite uptake at the gills and is a stopgap during a nitrite spike. Long-term solution is finishing the cycle.

Beginner-safe rule

Beginner-safe rule: if nitrite reads above 0.25 ppm, do a partial water change to dilute it. If you have fish in the tank, dose plain aquarium salt as a temporary protective measure. Don't stop dosing ammonia mid-cycle just because nitrite spikes — that stalls the cycle.