Fish preparation Nile tilapia of mixed sexes were obtained from
the Nong Khon Farm . To reduce stress induced by environmental changes on fish, they were maintained in 150 L [Remark 6] plastic tanks at 25 °C, under a 12 h light:12 h dark photoperiod, and fed commercial fish feed (C. P. Classic; S. W. T. Co., Ltd., Bangkok, Thailand) for 2 weeks prior to the experiments. Just before the experiments, fish were randomly selected and tested for presence of bacteria in their livers and kidneys on BHI agar.