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AquaReefBasin

Parameter

Nitrite

nitrite ion, the intermediate product between ammonia and nitrate

Reported in ppm nitrite

Where a reading sits on this scale

  • Undetectable

    0 ppm

    In range

  • Trace

    0.02 – 0.25 ppm

    Borderline

  • Present

    0.25 – 1 ppm

    Act now

  • High

    above 1 ppm

    Act now

What it is

Nitrite is what the first group of nitrifying bacteria make out of ammonia. It is a waypoint, not an endpoint: a second, slower-growing group converts it onward into nitrate. In a tank that has finished cycling, nitrite never accumulates, because the second group has grown large enough to keep pace with the first. In a tank that is still cycling, or one whose filter has been damaged, nitrite piles up in the gap between the two populations.

Why it matters

Nitrite interferes with the blood's ability to carry oxygen, which is why fish in nitrite-heavy water often sit near the surface or near the filter outflow and breathe hard even though the tank is well aerated. It is also the most reliable signal that a cycle is partway done rather than finished — ammonia falling to zero while nitrite climbs is the classic middle-of-the-cycle picture, and it is the point where people declare victory too early and add fish.

What each band means

Nitrite bands, in ppm nitrite
BandRangeStatusWhat it means
Undetectable0 ppmIn rangeThe second bacterial group is keeping up. In an established tank this reads zero permanently, including right after a feeding.
Trace0.02 – 0.25 ppmBorderlineCommon in the back half of a cycle, and a warning in a tank that was reading zero last week. Test ammonia at the same time — the pair tells you which end is struggling.
Present0.25 – 1 ppmAct nowWater change today, and no feeding until it comes down. This is the reading that goes with hard breathing at the surface.
Highabove 1 ppmAct nowThe cycle is not finished or the filter has lost most of its colony. Repeated partial changes until it holds at zero for several days in a row.

How to test it

  1. Always test nitrite alongside ammonia. Either number alone is ambiguous; together they place you on the cycle.
  2. Test at the same time of day. Nitrite in a cycling tank moves on a daily rhythm tied to feeding, and comparing a morning reading to last night's tells you nothing.
  3. Rinse the test tube with tank water before filling it. Residue from yesterday's nitrate test will skew a nitrite reading.
  4. Nitrite colors are pink-to-purple and go very saturated very fast. A deep purple tube is off the top of the card, not the top value on it — dilute the sample with a known volume of distilled water and multiply.

How to move it

  1. Change water. Nothing removes nitrite faster and nothing else removes it at all.
  2. Stop feeding until the reading holds at zero. Every feeding adds ammonia at the front of the chain and pushes more nitrite out the middle.
  3. Leave the filter media alone. The bacteria you need are living on it, and rinsing or replacing it now extends the stall by weeks.
  4. Add aeration — raise the outflow, add an air stone. It does not remove nitrite, but it makes the oxygen situation less punishing while the reading comes down.
  5. If you are cycling deliberately, be patient rather than clever. The second bacterial group is genuinely slow, and most stalls are impatience rather than a real fault.

Where the reading goes wrong

  • Reading a saturated purple tube as the top card value. Past the top of the scale the color stops changing, so a very high reading looks identical to a moderately high one.
  • Testing straight after a water change and concluding the tank has cycled. You diluted the number; you did not grow bacteria.
  • Using strips near the end of their life. Nitrite pads are among the first to degrade once the tube has been opened and left in a humid cabinet.
  • Confusing the nitrite and nitrate bottles in a master kit. They sit next to each other, the labels differ by two letters, and the mix-up is extremely common.

Problems that start here

Published sources

Published figures on this page come from the documents above. Where a value depends on species, pH or temperature, that is stated rather than averaged into a single number.