Nitrite: the Deadly One

for the week of 12/2/99

Many aquarists confuse nitrite (with an “i”) with nitrate (with an “a”). They are chemically quite similar (the difference being an extra oxygen atom in the nitrate ion), but nitrite is far more toxic in low concentrations than nitrate.

Once absorbed by fish, nitrite combines with the hemoglobin in blood, eliminating its ability to carry oxygen. The fish can thereby suffocate even though there may be plenty of oxygen in the surrounding water. Some types of hardy freshwater fish (including many of the tetras, barbs and danios we recommend as “starter” fish) appear to be unaffected by nitrite. Also, the addition of one tablespoon of aquarium salt per five gallons of water seems to reduce the uptake of nitrite by most other species. In recent years, there has been some thought that many saltwater fish are similarly protected from nitrite poisoning, but it is of course wise to err on the side of caution.

Nitrite is generally only a problem during the “break-in” process of the first month of a new tank, or the weeks following an overly thorough-tank cleaning. In a stable, mature aquarium, nitrite is rapidly converted to the less toxic nitrate by bacteria, and it would be abnormal to see even a trace of nitrite when testing an established tank.

An established tank with even trace readings of nitrite indicates a major problem and impending disaster. The possible causes include:

  • Overcrowding – too many fish equals too much waste for bacteria to process.
  • Overfeeding – rotting food and excess waste also overload the biological breakdown process.
  • Removal of bacteria – usually by changing all the gravel, or too many other objects that bacteria have colonized.
  • Killing of bacteria – by overmedication, large water change, adding chlorinated tap water.
  • Old Tank Syndrome – by dropping pH in neglected aquarium.

“Tip of the week” appeared regularly in 1999 and 2000.