Abstract: The statistical experimental design (Plackett-Burman and Box-Behnken design) was applied to optimize the culture medium of nitrite oxidizing bacteria for improving the nitrite oxidizing rate. Estimated optimum medium composition of the nitrite oxidizing rate was as follows: NaHCO₃ 2.0 g · L^-1; NaNO₂ 2.36 g · L^-1; Na₂CO₃ 0.37 g · L^-1; NaCl 0.34 g ·L^-1; KH₂PO₄ 0.05 g · L^-1; MgSO₄ · 7H₂O 0.05 g · L^-1; and FeSO₄ · 7H₂O 0.03 g · L^-1. The nitrite oxidizing rate reached a maximum at 905.0 mg NO₂-N · (g MLSS · d)^-1(mixed liquor suspended solids, MLSS).In the field trial, 50 L of nitrite oxidizing bacteria concentrate (1.99 g VSS · L^-1)(volatile solid, VSS) with 850 mg NO₂-N · (g MLSS · d)^-1 were added to 0.6 hectares of the aquaculture water. Nitrite level in all treated ponds remained very low compared to the steady increase observed in all of the control ponds during 7 days.