The study aimed to evaluate the effect of Thepax and BioBoost as food additives on the microorganisms in the intestines of fish. From March 4 to October 22, the total number of common carp was 900 fish with an average weight of 163.41 ±10.16 g and a density of 100 fish/cage; three replicates were used for each treatment. The fish were fed three times daily. The included T1(0%additive), T2(1g/kgThepax)and T3 (1 g/kgBioBoost). The highest final weight value is Thepax treatment (2209.34 g), followed by Bio Boost and control. Microorganisms showed significant differences (P<0.05) in T2 for Lactobacillus sp. (10² x 65 CFU/ml) followed by T3 (10² x 55 CFU/ml) and control T1 (10² x 23 CFU/ml), also for Cellulomonas sp. in T2 (10² x 54 CFU/ml) followed by T3 (10² x 39 CFU/ml) and control T1 (10² x 7 CFU/ml). At the same time, Aeromonas sp. bacteria was higher in T1 (10² x 34 CFU/ml) over the treatments of T2 and T3 (10² x 2 CFU/ml) for both. We concluded the best additive was 1 g of Thepax / kg of feed in the recommended diets for common carp. Keywords: Thepax; Additives; Microorganisms; Intestines; Bacteria; Lactobacillus; Endo Bio Boost