It could be Methemoglobinemia. Nitrites are actively transported across the gills and readily oxidize hemoglobin to form methemoglobin. Methemoglobinemia results in hypoxia severe enough to cause sudden death but often the fish will live until they exert themselves. The term "brown blood disease" comes from the appearance of the blood that has high levels of methemoglobin (which is brown). Often, gross lesions are lacking, therefore; the brown appearance of the blood can be a diagnostic tool. Another diagnostic method involves measuring nitrite levels in the water. This may be unrewarding if mortality is high enough to decrease fish density and subsequent nitrite levels. Nitrite levels should not exceed 0.10 mg/1 in channel catfish or 0.50 mg/1 in salmonids. The LC50 for the majority of freshwater fish ranges from 0.60 to 200 mg/1. Saltwater fish have a much higher tolerance for nitrites. Treatment not only includes decreasing the population to decrease ammonia levels, but also adding a chloride salt (in the form of sodium chloride or calcium chloride) to the water. The level of salt needed to treat (<50 mg/1) is not toxic to freshwater fish. The chloride ion competes with the nitrite ion at the gills. When the chloride ion is present at least three times and not more than six times the level of the nitrite ion, it is preferentially transported across the gills. Thus transport of the nitrite ion is reduced. Keeping the chloride levels in the water at least 20 mg/1 can prevent nitrite toxicosis. Additional treatments can include emergency water changes to dilute the nitrite problem.
