Nitrite (NO₂)

What it is

Nitrite is the second stage of the nitrogen cycle, produced when one group of bacteria oxidises ammonia and consumed when a second group converts it into nitrate. In a mature, cycled tank the second group keeps pace with the first, so NO₂ never accumulates and the reading sits at 0 mg/L — the only acceptable steady-state value. Anything above 0.1 mg/L means biological filtration is partially broken; above 0.5 mg/L is acutely dangerous, and past 1 mg/L it will kill sensitive fish within a day. Like ammonia and nitrate, it's measured in mg/L (equivalent to ppm).

Why it matters

Nitrite gets into the bloodstream through the gills and binds to haemoglobin, forming methaemoglobin, which can't carry oxygen — the reason the condition is nicknamed "brown blood disease". Effectively the fish suffocate in fully oxygenated water. At 0.5 mg/L fish are visibly stressed, gilling fast and hanging at the surface; at 1 mg/L sustained, mortality starts within hours to days. Shrimp and other invertebrates tolerate even less. There's a useful quirk in the chemistry: chloride ions compete with nitrite for uptake at the gill, which is why a dose of plain aquarium salt can protect fish during a spike (more on that below). A non-zero NO₂ reading in an established tank always means something is wrong — it should be the exception that triggers an investigation, never a number you learn to live with.

What a nitrite spike looks like

Fish gill rapidly and hang near the surface or right at the filter outflow, chasing oxygen they can't use. They go lethargic, lose appetite, and may sit on the bottom with clamped fins; in pale fish the gills can take on a brownish cast as the methaemoglobin builds. Livebearers, corydoras and other bottom dwellers often show it first. It looks a lot like ammonia poisoning, which is no coincidence — the two travel together during a cycle, so if nitrite is up, test ammonia as well. Nitrite is a livestock problem, not a plant one; plants are largely indifferent to it.

Where a nitrite spike comes from

In a new tank, nitrite is the second peak of the cycle — it climbs only after the ammonia-eaters get going, then falls as the nitrite-eaters (Nitrospira) catch up. That mid-cycle spike is normal and expected. In an established tank it means the second-stage colony has been knocked back: a power cut or pump failure that starved the filter of oxygen, rinsing all the media in tap water at once, replacing the whole filter cartridge, a course of antibiotics that wiped the bacteria, or a sudden jump in bioload the colony can't match. Track down which of those happened — the fix is the same, but knowing the cause stops it recurring.

How to fix a nitrite spike

Don't wait for the bacteria to "catch up" while fish are stressed. Do a 50% water change immediately to dilute the NO₂, and repeat the next day if it's still above 0.1 mg/L; the water-change impact tool shows how far each swap moves the number. As first aid, add aquarium salt (sodium chloride) at roughly 1–3 g per litre — the chloride blocks nitrite uptake at the gill and buys the fish time, though check your species tolerate salt first (many soft-water and scaleless fish don't love it). Boost aeration, since fish fighting nitrite need all the dissolved oxygen they can get. Cut feeding to a quarter so less ammonia enters the front of the chain, and skip any new livestock until you see consecutive zero readings. If the filter genuinely crashed, reseed it with bottled bacteria or mature media. The nitrogen cycle in plain language explains why the nitrite step lags the ammonia step, and the nitrogen-cycle timeline tool gives a rough sense of how long recovery takes.

Testing notes

Liquid drop tests are standard (JBL NO₂, Tetra NO₂, Salifert). One real pitfall at high concentrations: some kits saturate and read falsely low or stall at an odd colour when nitrite is very high — if you suspect a big spike but the tube shows a middling result, dilute the sample 1:1 with distilled water, retest, and double the reading. Test immediately when you suspect trouble — fish gasping, cloudy water after a disturbance, a dose of medication, after disturbing the substrate — and daily throughout cycling. In a mature tank you can drop to monthly. Read nitrite as part of a set: it sits between ammonia and nitrate, so a spike in the middle tells you exactly where in the chain the problem is.