Nitrite Toxicity in the Aquarium
Ardan Huck July 11, 2002

Nitrite is a byproduct of Nitrosomonas bacteria breaking down ammonia during the nitrogen cycle. Nitrobacter bacteria break down Nitrites into nitrate.

If levels are too high in an aquarium it can affect the fish's respiration. Generally, levels of nitrite below 2 mg/l are considered safe for fish. Nitrite toxicity is somewhat dependent on the pH of the water and becomes more toxic in acidic conditions (reverse of ammonia toxicity). If the pH is above 7, discus can tolerate higher levels, but the amount varies on the fish (depends on size, age, and health).

Water changes are often used to reduce the nitrite levels in the aquarium, however, changing 90% of the water will only reduce the nitrite concentration by 30% (Kohler, 1997). It takes Nitrobacter bacteria about 30 hours to reproduce and double the size of the colony of bacteria. This is somewhat dependent on temperature and pH of the water. Bacteria will reproduce and grow faster at higher temperatures and higher pH (to a point).

Another way to control nitrites is to reduce feeding, as the less organics (nitrogen compounds) introduced into the aquarium result in less ammonia/nitrite, however, removal of all organics (food and fish which give off waste containing nitrogen), would starve the bacteria and the bacteria would die.

Salt added to the aquarium, usually 2 tablespoons per 10 gallons water, helps the fish because the chloride ion (cl-) from the NACL (sodium chloride, or salt) attaches to the gill cells where oxygen enters the blood. This is also the place where nitrite enters the blood, therefore the chloride blocks the nitrite from entering. This only works well if the pH is above 7.0 as in acidic conditions Nitrite (NO2) will bind with hydrogen ions, H+ (acid contains more H+) and form Nitrous acid (HNO2). Nitrous acid is not blocked by CL- ions at the gill site, and thus can pass freely into the blood.

Nitrite combines with the iron in the hemoglobin (changes it to methemoglobin, called "brown blood"), oxidizing the iron and changing it from Fe+2 to Fe+3. Fe+2 will bind with oxygen as is normally occurring during respiration; Fe+3 cannot bind with Oxygen (O2).

First signs of nitrite toxicity may include increase respirations or gasping for air, and possibly darkening in color. They then become lethargic (slow) and can die.

"DiskusBrief" magazine, Vol IV, Issue II, 1997, "Nitrite and Nitrate in the Aquarium" by Horst W. Kohler and Paul Ceroke

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