The present study was carried out to clarify the effect of adding Bacillus subtilis as a probiotic and growth promoter, to evaluate its potential for Oreochromis niloticus fingerlings growth performance. Used fish were divided into two groups according to their initial body weight; small size fish with average initial body weight 3.5 g. and length 3.7 cm. and big size fish with aver. IBW 15.3 g. and length 13.5 cm. Each fish group, was divided to four subgroups; T1 was the control that offered the basal diet only, while the other three subgroups ; T2 , T3 and T4 were fed on basal diets that contained B. subtilis solution with three different levels 5,10 and 15.0 ml /l00 g diet, contained 108 CFU ml- 1 respetively. Experimental period was 90 days in a three replicates. The results show that growth performance in small size O. niloticus (FBW), (WG), ADG and SGR values increased significantly with increasing B. subtilis levels. Differences, were observed with significantly higher values,recorded from the highest B. subtilis level, comparina to the control. Feed
conversion rate showed decreases with the increasing B. subtilis level. Survival had best values for fish groups offered diets with adding B. subtilis. Serum analysis, total protein, albumin and globulin had the same pattern. Maximum values were recorded from T4 in contrast to the control (T1). Big size fish showed same results of growth performance, with significant differences between fish groups offered diets with B. subtilis and the control diet groups. Differences were significantly higher from treatment (4) compared to the control. F.C.R showed the lowest value from T4, while T1 recorded the highest value. Big size fish showed the same results in FBW, WG, ADG and S.G.R. Differences were observed with significantly higher values from fish groups offered diet contained high B. subtilis level. Compared to the control, F.C.R. showed the best and lowest value from treatment (4), in contrast to the control which recorded the highest value.Serum analysis (T.P.C), (A) albumen and (G) globulin increased significantly with the increase in B. subtilis level. Treatment (4) had the best values in contrast to the control. In both small and big size fish. A: G rate
decreased with the increasing B. subtilis level, best value was recorded from treat. (4), while T1 recorded the highest value. Small size fish had the best results and were more benefited from adding B. subtilis to their diets than those in big size fish. These results indicted that B. subtilis can be used effectively as a commercial product for enlrancing growth performance in aquaculture.