Immature zebrafish (0.2-0.5 g) were purchased from a local pet shop. They were acclimated to the experimental fresh water in a 15-l plastic aquarium, with a water flow of 1.3 l h^(-1), for 2 months before the experiment. The soft fresh water was mixed 24 h before usage by adding salts to deionized water in 110-l tanks (Table 1) . Fish were fed commercial fish food twice a day and the water temperature was 26±1℃. The fish were subjected to a photoperiod of 12 h light /12 h dark.