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Ammonia is reduced by a bacteria called “Nitrosomonas” into Nitrite. These Nitrites are toxic at very low levels. People often test 0.25 (a quarter) part-per-million of Nitrite and disregard it as a problem. “Oh, that’s not enough Nitrite to matter!” they say. Perhaps they are fools. Their fishes’ health problems never end. Not until they reduce the Nitrites to conventionally immeasurable levels. Sure, there are tests which can detect traces of Nitrite at all times. That’s normal. But what you’re looking for is a “negative” result with the commercially available Nitrite test kit.
If you find Nitrites in a recently-populated system, it’s pretty normal. The bacteria which will eventually colonize your filter and the surfaces of the pond-bottom will not have had time to colonize. You can speed it up with Fritz TurboStart. I have tested this one extensively and found it to work very well. If you find Nitrites in a well-established older system, it usually means the filter is being deprived of sufficient oxygen and / or the filter is clogged with organic waste products. The filter media should be gently cleaned with pond water and you should make sure the filter is sized correctly to your fish load.
Nitrites hurt the fish two ways. First, the nitrites are caustic (they burn) the fishes’ skin. Also, the Nitrites pass into the fishes bodies through the gills and create a damaging bond with the red blood cells. The situation is called met-hemoglobinemia wherein the red blood cells can no longer carry oxygen. THe blood runs brown instead of red. The fish dies shortly after this.
Did you know that you can slow (and even STOP) the absorption of Nitrites from the water with salt? If you use as little as one pound of non iodized table salt, or regular fish-safe salt per one hundred gallons, it interferes with the passage of Nitrite back into the fishes’ gills. This effect is not permanent but usually lasts long enough for the beneficial bacteria to populate your filter and bio-film and reduce the Nitrite. When nitrite levels are very high, a partial water change is also helpful if it’s possible to do.
You’ll read about methylene blue being used to stop Nitrite intoxication. This is because veterinarians used to use methylene blue in cows for nitrite and nitrate poisoning. They’d inject it into the cow intravenously. I guess someone thought if it worked for cows it would have to work in the water with fish. No, it doesn’t. Save yourself the trouble.
SHORT AND SWEET: Nitrites “pop up” just after the Ammonia begins to decline in the water and are best controlled with limited salt application, OR partial water changes, and a bacterial adjuvant.