Mixed culture of nitrifying bacteria from RAS biofilters was immobilized on plastic media after 45 days incubation in shrimp culture tanks. This was following by NOB selection in nitrite broth under laboratory conditions.The desired condition for NOB selection is presented in Table 1 (Hart and O’sullivan, 1993; Lawson, 1995; Satoh et al., 2000; Timmons and Losordo, 1994). The activity of selected NOB on plastic media is shown in Fig. 2. It was found that nitrite concentration in the water was reduced to 0.20 mg-N/L within 8 hr and nitrite removal rate was about 28.09 ± 0.58 mg-N/(m2·day).Consequently, nitrate concentration in the test chamber increased in proportion to the decrease of nitrite while ammonia remained constant. This indicated the complete nitrite oxidation process.