Nitrite, NO2-N, measured in parts per million (ppm), is the second chemical measurement made to determine the "health" of the biologic converter. Nitrite should not be detectable in a pond with a properly functioning bio-converter. Thus the ideal and normal measurement of nitrite is zero. A low nitrite reading combined with a significant ammonia reading indicates the ammonia-nitrite biologic converter action is not established, while a low ammonia reading with a detectable nitrite reading indicates that the nitrite-nitrate bacterial conversion activity is not yet working. Test kits are available in pill, powder, or droplet forms with color charts. Recommended test kit range 0 - 4 ppm. A nitrite test kit is considered to be a requirement for all pond keepers.
Source: Nitrite is produced by autotrophic bacteria combining oxygen and ammonia in the bio-converter and to a lesser degree on the walls of the pond. Just as with ammonia, nitrite readings may increase with a sudden increase in bio-converter load until the bacterial colony grows to accept the added material. This can happen following the addition of a large number of new fish to a pond or during the spring as the water temperature increases. Fish activity can often increase faster following a temperature increase than the bacterial action does. A bio-converter that becomes partially obstructed with waste and/or develops channels through the media may operate at reduced effectiveness that can also cause the nitrite levels to increase.
Effects: Nitrite has been termed the invisible killer. The pond water may look great, but nitrite cannot be seen. It can be deadly, particularly to the smaller fish, in concentrations as low as 0.25 ppm. Nitrite damages the nervous system, liver, spleen, and kidneys of the fish. Even lower concentrations over extended periods can cause long term damage. Short term, high intensity, "spikes" which often occur during a bio-converter startup or restart may go undetected yet cause problems to develop within the fish months later. A common indication of a fish that has endured a severe nitrite spike in the past is that the gill covers may be slightly rolled outward at the edges. They do not close flat against fish's body.
Control: About the only control of nitrite is through the maintenance of a "healthy" bio-converter. Within the media, the bacteria combine oxygen with the nitrite to convert it to the relatively benign nitrate. These bacteria receive considerably less energy from this conversion process than do the bacteria carrying out the ammonia to nitrite conversion process. For this reason, they are not as hardy and tend to be the last to come and the first to go when a problem occurs within the bio-converter. Water change outs can reduce the levels temporarily by the same amount as the percentage of water changed. The addition of salt helps reduce the toxic effects significantly but should only be used as a interim measure, not as an ongoing treatment.
Whenever 0.25 ppm of nitrite or more is detected in a pond:
Increase aeration to maximum. For a nitrite level of 1 ppm or greater, add supplemental air, if possible.
Stop feeding the fish if detected in an established pond, reduce amount being fed by half if starting up a new bio-converter/pond.
Discontinue use of any UV Sterilizers, Ozone Generators, and Foam Fractionators (Protein Skimmers).
For a nitrite level less than 1 ppm, conduct a 10% water change out and add 1 pound of salt per hundred gallons of changed water.
For a level between 1 and 2 ppm, conduct a 25% water change out and add 2 pounds of salt per hundred gallons of changed water.
For a level greater than 2 ppm, conduct a 50% water change out and add 3 pounds of salt per hundred gallons of changed water.
Retest and repeat above in 24 hours.
For nitrite levels of 4.0 or greater, consider transferring fish.
CAUTION: Again, if the added water contains Chloramine, the added ammonia after conversion to nitrite may make the situation worse.