Fish preparation Nile tilapia (O. niloticus) of mixed sexes were obtained from
the Nong Khon Farm (Ubon Ratchatnai, Thailand). 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.