Nitrite has been shown to be highly toxic to grass carp. It can in-duce hemoglobin (Hb) to become methemoglobin (MHb). If methemoglobin isover 23.0% of total Hb, the grass carp will easily get hemorrhage disease. The ex-periment shows that 96-h LC₅₀ values of NO_2-N was 4.62mg/L at the average watertemperature 30℃, especially over 32℃. As the number of methemoglobin increasesin the blood of grass carp, the number of erythrocyte and total hemoglobin gra-dually decrease. The way to remove nitrite-nitrogen from water is by adding so-dium hypochlorite. When the concentration of NO₂-N is over 0.15 mg/L, at firstcalcium hydroxide [Ca(OH)₂] is added before the addition of sodium hypochlorite.