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Aquaculture is inextricably linked to fish health problems, and the use of probiotic lactic acid bacteria (LAB) makes it possible to reduce losses caused mainly by bacterial pathogens. The aim of the present study was to isolate, characterize and select potential probiotic strains of lactic acid bacteria from the gut of common carp. We focused on obtaining strains that grow well under aerobic conditions to facilitate their future application through feed. After excluding strains with unwanted characteristics in vitro, two isolates of Levilactobacillus brevis were selected for this study. Despite belonging to the same species, the strains were biochemically different. However, both isolates met the in vitro selection criteria for potential probiotics. The tested L. brevis strains showed antagonistic activity against pathogenic bacteria isolated from carp and rainbow trout (Aeromonas hydrophila, Aeromonas salmonicida, Pseudomonas fluorescens, and Acinetobacter junii) and demonstrated co-aggregation ability with pathogens. Both strains tolerated stress well under simulated gastrointestinal tract conditions (i.e., showed acid and bile tolerance) and exhibited auto-aggregation capacity. The tested isolates were nonhemolytic, gelatinase negative and susceptible to most of the antibiotics tested. The cell-free supernatants of both L. brevis isolates exhibited strong immunostimulatory effects on pikeperch head kidney immune cells and immunomodulatory effects on carp immune cells. The results of in vitro tests encourage further research into the probiotic potential of these strains under in vivo conditions.