Nitrite

After ammonia, nitrites are the next toxic compound you need to watch for in your aquarium. Nitrite (NO₂⁻) is the intermediate product in the nitrogen cycle, produced when beneficial bacteria oxidize ammonia. Ideally, it should never accumulate—if you’re seeing nitrites on a test, your cycle is not complete or has broken down.

In freshwater, nitrite is especially dangerous because it mimics chloride (Cl⁻) ions, hijacking the same transport pathways into the fish’s bloodstream. Like carbon monoxide in mammals, nitrite binds to hemoglobin, preventing oxygen from being transported properly. A common symptom of nitrite poisoning is fish gasping at the surface despite adequate oxygen in the water.

Note: There should never be nitrite in your tap water. If you’re detecting it, the source is likely the aquarium's own waste breakdown process.

Treating High Nitrites

• Water Changes

  • As with ammonia, a 50% water change is the fastest way to reduce nitrite levels. Repeat daily if needed until nitrites reach zero.

• Strengthen Your Filter

  • Check your filtration system:

    • Is it new or not fully cycled?

    • Did you change out or rinse the media?

  • Use established filter media from another tank or a small amount of clean soil to introduce the correct bacteria to process nitrites into nitrates.

• Add Salt to Reduce Toxicity

Unlike ammonia, salt can directly help protect fish from nitrite poisoning in freshwater tanks. By increasing chloride levels in the water, salt competes with nitrite for uptake pathways, effectively blocking its absorption into the fish’s bloodstream.

For general prevention or during a fish-in cycle, use aquarium salt, not table salt. Most freshwater fish can safely handle salinity up to 1.005 SG (about 5 teaspoons or 30g of salt per gallon). While this amount is best reserved for dips or treatments, a modest amount of salt can act as a buffer against nitrite exposure during cycling.