Tilapia is a popular food fish all over the world and in Egypt it has been raised as food for human consumption for a long time. A field study was conducted on brood stock Nile tilapia Oreochromis niloticus to improve their reproductive performance by some commercial feed additives. Both sexes were individually stocked into hapas (enclosures) and fed a basal diet supplemented with graded levels 0.0, 0.5, 1.0 and 2.0 g Therigon® kg-1 diet (T1-T4; T1 as a control treatment, respectively); 1.0, 2.0, and 3.0 g Nuvisol Hatch P® kg-1 diet; (T5-T7); 20, 40 and 60 mg gibberllic acid kg-1 diet (T8-T10) for female only and 700, 900 and 1100 mg L-carnitine kg-1 diet for males only (T12-T14), respectively for 19 day. Results revealed that fish ovaries in all treatments (T2-T10) contained more advanced oocytes at final maturation stage with clear histological alterations of fish fed the highest levels of these feed additives, especially with gibberllic acid treatment. Fish treated with 60 mg gibberllic acid kg-1 diet (T10) showed the worst histological effects on fish ovary among all treatments. Addition of 700 and 900 mg L-carnitine kg-1 diet caused maturation of testes better than the control treatment (T11). Meanwhile, fish fed 1100 mg L-carnitine kg-1 diet (T14) showed degeneration and severe hemolysis in seminiferous tubules. Low bacterial loads were recorded (in gills, liver, intestine and ovary / testes) at the high level of Nuvisol Hatch P®. Data proved that the high bacterial load was detected in fish treated with L-carnitine in both tested media and in all tested organs. Meanwhile, low bacterial load was observed in the liver, ovary and testes in all media comparing with the other organs (gills and intestine).