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As you know, Nitrite is part of the nitrogen cycle and occurs when ammonia is converted to Nitrite. Nitrite can oxidize hemoglobin in red blood cells and convert it into a compound (methemoglobin) that cannot carry oxygen. Normal hemoglobin carries oxygen through the bloodstream, but methemoglobin cannot. Nitrite enters the blood stream of fish through the gills and turns the blood a dark brownish color, thus the name 'brown blood.' As a result and as Chuck described, affected fish show signs of low oxygen stress such as gasping at the surface or hanging near aeration equipment/waterfalls. The affected fish exhibit these signs even when the dissolved oxygen levels in the water are normally adequate. The intensity of this condition can be judged by the color of the fish blood. Slightly affected fish have reddish-brown blood, and severely affected fish have chocolate brown blood. The percentage of the hemoglobin that has been converted to methemoglobin combined with the dissolved oxygen concentration in the water will determine the number of fish that will/could be effected and die. For example, fish moderately affected with 'brown blood' disease in a pond with a DO reading of 7 ppm should/could survive, however, the same fish in a pond with a DO reading of 2.0 ppm may not. Nitrite toxicity is affected mainly by temperature, dissolved oxygen, and chloride ions. A nitrite concentration as low as 0.5 ppm can cause stress, so it's obvious that zero ammonia and zero nitrite is the goal to achieve in ones water quality.
The answer to alleviate this condition (brown blood disease) is salt (NaCl), and more specifically, the chloride (Cl) in salt, which is the portion of salt that is responsible for the prevention and treatment of 'brown blood' disease in fish. Chloride competes with nitrite at the fishes gill surface for absorption. If a 10 to 1 ratio of chloride to nitrite is maintained in water, many more chloride ions are absorbed than are nitrite molecules and, therefore, 'brown blood' disease does not develop. You can actually calculate how much chloride is needed to absorb the nitrite level one may be experiencing. I won't go there but tell you that the 'normal' 0.3% salt level is more than enough to prevent brown blood disease.
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