Nitrite (NO₂)
The toxic middle step of the nitrogen cycle. In an established tank it should always read zero.
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 up with the first and NO₂ never accumulates.
Why it matters
Nitrite binds to fish haemoglobin and blocks oxygen transport — at 0.5 mg/L fish are visibly stressed, gulping at the surface; at 1 mg/L sustained, mortality starts within hours to days. Shrimp tolerate even less. A non-zero NO₂ reading in an established tank always means something is wrong: a filter crash, a too-deep clean, a new bioload spike, or an antibiotic dose that wiped the second-stage bacteria.
How to test
Liquid drop tests are standard (JBL NO₂, Tetra Test NO₂, Salifert). Test immediately when you suspect a problem — fish gasping at the surface, sudden cloudy water, a dose of medication, after disturbing the substrate. In a mature tank you can drop NO₂ testing to monthly; in a cycling tank, daily.
What high and low look like
Zero is the only acceptable steady-state reading. Anything above 0.1 mg/L means biological filtration is partially broken. Above 0.5 mg/L is acutely dangerous; above 1 mg/L will kill sensitive fish within a day.
How to fix
Don't wait for the bacteria to "catch up". Do a 50% water change immediately to dilute the NO₂ — repeat the next day if it's still above 0.1 mg/L. While the filter recovers, reduce feeding to a quarter (less ammonia in = less NO₂ produced) and skip any new livestock until consecutive zero readings return.
