This study evaluated the antibacterial activity of Origanum majorana essential oil (EOM) and nanocapsules of this oil (NOM) in silver catfish, Rhamdia quelen, infected with Aeromonas hydrophila, and addressed their effects on silver catfish hematological and metabolic parameters. Fish were inoculated with A. hydrophila (360 μL, at a concentration of 1.5 × 109 CFU mL-1) and submitted to 1 h daily baths with EOM (0 (control), 20 or 30 μL L-1), NOM (0 (control), 5 or 10 μL L-1) or a positive control containing florfenicol (30 μL L-1) called group Maxflor® for five consecutive days. All treatments improved the survival rate of the infected fish, but we suggest the treatment of A. hydrophila infections through daily baths with 20 μL L-1 EOM or 5 μL L-1 NOM for five consecutive days as these were the lowest effective concentrations tested. Silver catfish treated with EOM and NOM had higher lymphocyte levels, indicating stimulation of the immune system in these fish. The lowest liver glucose level was found in the group treated with the lowest concentration of NOM, and the lactate values in the liver and muscle of all groups were within the normal values reported for this species. In addition, nanocapsules required much less EOM to elicit effective antibacterial treatment.