This study aimed to evaluate the potential of Lippia sidoides essential oil as a food additive for Cyprinus carpio koi carp in improving the health parameters and resistance to infection by Aeromonas hydrophila. A total of 312 carp were divided into groups. Twenty-four animals were in the lethal dose 50% (LD50) test group. The remaining 288 were distributed in 24 experimental units (n = 12) divided into six treatments-Lippia0.250%, Lippia0.125%, Lippia0.063%, Lippia0.031%, alcohol, and control-with four replicates each. The animals were fed thrice daily with 5% of living biomass for 55 days. At the end of the period, five specimens were collected from each treatment to analyze the intestinal tract's zootechnical performance, hematological analysis, and microbiology. The remaining animals were challenged with A. hydrophila, and the mortality rate was monitored for 100 days. Thymol was the component with the highest concentration (76.6%) in the essential oil of L. sidoides. Fish fed Lippia0.125% had the greatest post-challenge survival. Fish fed Lippia0.063% showed increased zootechnical performance, and those fed Lippia0.250% had the highest concentration of lactic acid bacteria in the intestine. Hematological analyses did not show significant differences among treatments. The authors suggest new studies with higher concentrations of L. sidoides essential oil in the diet of C. carpio than were used in this study.