Nitrite in a fish tank: what it means and how to fix it

Nitrite is the middle step of the nitrogen cycle, and it's toxic to fish. In a healthy, cycled tank it should always read zero. If your test shows any nitrite, here's what's going on and what to do today.

Safe nitrite levels

Why nitrite is harmful

Nitrite gets into a fish's blood and stops it from carrying oxygen. Fish with nitrite poisoning gasp at the surface, breathe fast, sit still and may have brownish gills, even when the water is full of oxygen. See fish gasping at the surface.

Where nitrite comes from

Fish waste and old food make ammonia. One kind of good bacteria turns ammonia into nitrite, and a second kind turns nitrite into nitrate, which is much safer. Nitrite shows up when the second group of bacteria can't keep up:

• A new tank that isn't cycled yet. This is the most common cause. See how to cycle a new fish tank.
• Too many new fish at once, making more waste than the bacteria can handle.
• Overfeeding. See how often to feed fish.
• A filter cleaned too hard or with tap water, which kills the bacteria. See how to clean a filter.
• Medicines that harm good bacteria, or a filter that was switched off for hours.

How to fix high nitrite

1. Change 30 to 50 percent of the water with dechlorinated water at the same temperature. See how to do a water change.
2. Use a conditioner that detoxifies nitrite, such as Seachem Prime, which helps protect fish for about a day between water changes.
3. Feed very lightly, or skip a day or two. Fish are fine without food for a few days.
4. Don't add any more fish until nitrite reads zero.
5. Add bottled bacteria to help the filter catch up.
6. Test every day and repeat the water change until both ammonia and nitrite read zero.

Some fishkeepers add a small dose of aquarium salt, because the chloride in it blocks nitrite from entering a fish's gills. Follow the label, and skip it with corydoras, loaches and live plants, which are sensitive to salt.

How long will nitrite last?

In a new tank, nitrite usually peaks a week or two after ammonia and can take 2 to 4 weeks to drop to zero. In an established tank, a spike from overfeeding or a filter mistake usually clears in a few days with water changes.

Nitrite, nitrate and ammonia

• Ammonia: toxic, should read 0. See ammonia in a fish tank.
• Nitrite (with an i): toxic, should read 0.
• Nitrate (with an a): much less harmful, keep it under 40 ppm. See high nitrate.

Related: Ammonia in a fish tank · How to cycle a fish tank · Why are my fish dying?