Each extract was tested in triplicate. Negative controls were prepared in the same way except that 40 μl of pure solvents were used instead of the herb extracts. Oxytetracycline (5 μg on sterile paper disc), a common antibiotic used in tilapia aquaculture in Thailand, served as the positive control. The prepared agar plates were then incubated at 25 °C for 18 h,