Here's something I have found...some of it applies to this thread, but also to a previous thread on salt treatments. For simplicity, I have cut n pasted.

The second breakdown product in the nitrogen cycle is NO2, which is also toxic to fish. NO2 can enter the bloodstream passively across the gill epithelium. It complexes with hemoglobin to form methemoglobin, resulting in methemoglobinemia or brown blood disease. As in other species, RBC containing methemoglobin are unable to transport oxygen, resulting in a physiologic hypoxia regardless of oxygen content in the water. There are species-specific differences in fishes’ susceptibility to NO2 toxicity (eg, centrarchids [bass, bluegill, etc] are refractory to methemoglobinemia). Marine fish were thought to be protected from NO2 toxicity by salts in their environment; however, red drum have developed brown blood disease in the presence of NO2. A tentative diagnosis of brown blood disease can be made by observing the characteristic chocolate brown color of the gills. Blood samples will also be an abnormal color. Methemoglobin concentrations in the blood can be determined, although this is not necessary for clinical management. A water quality test can confirm the presence of NO2. Fish affected with methemoglobinemia typically show signs of hypoxia, often manifest by piping. The most rapid treatment for NO2 toxicity is a water change, but this may not be feasible in production ponds. Increasing chloride Cl-) concentration in the water creates a competitive inhibition at the gill epithelium between Cl- and NO2. By increasing the concentration of Cl-, so that there are 6 parts Cl- to 1 part NO2, the percentage of Hgb converted to methemoglobin will decrease dramatically, providing immediate relief to the fish and stopping most further mortality within 24 hr. Salt can be used to increase the concentration of Cl-. To determine the amount of salt required, the concentrations of NO2 and Cl- present must be measured, by commercial test kits. The concentration of Cl- needed (mg/L) = (6 ¥ NO2) ? Cl- present. Once the concentration of Cl- needed is known, the volume of water can be calculated in acre-feet (1 acre foot = 1 surface acre, 1 foot deep), and salt can be added to increase Cl- to the desired concentration (4.5 lb of salt will add Cl- at 1 mg/L to 1 acre-foot of water). In aquariums and garden ponds, a water change and filter maintenance are recommended, although salt may still be used to halt mortality.