The article presents the results of evaluation of the effectiveness of the drug "Flavomycin 80" in feed for rainbow trout Kamloops breed when grown in a plant with a closed water supply system. It is shown that this feed additive leads to an increase in the viability of trout yearlings and an improvement in fish breeding and biological indicators of fish. The use of the drug at a dosage of 44 mg/kg contributed to a decrease in the intensity of damage to trout by pathogens of bacterioses of various etiologies. The positive bacteriostatic effect of the feed additive Flavomycin 80 on the development of conditionally pathogenic intestinal microflora of fish has been proven.