*3.1. Growth Performance*

The statistical results show significant differences in fish growth performance among the experimental groups (*p* < 0.05, Table 1). The WGR and SGR of the coral trout in the 2.5 bl/s water flow velocity group were significantly lower than those in the other groups (*p* < 0.05), showing also significantly higher FC values than the control and 1 bl/s water flow velocity groups, the lowest feed conversion rate, and the lowest weight gain. The VSI of fish in the 2 bl/s water flow velocity group was the lowest, significantly lower than that of the control group (*p* < 0.05) and not significantly among the water flow velocity groups (*p* > 0.05). There was no significant difference (*p* > 0.05) among the water flow velocity groups as regards the CF and HSI of coral trout.


**Table 1.** Effect of water flow velocity on growth performance of coral trout.

The values are the mean ± standard deviation of four replications (*n* = 6). Values with different letters imply a significant difference among the groups (*p* < 0.05), and no letters means no significant difference (*p* > 0.05).

#### *3.2. Blood Biochemistry*

Water flow velocity significantly affected the blood biochemistry of the fish (Figure 2). The GLU concentrations of coral trout were significantly higher in the control and 1 bl/s groups than in the 2 bl/s and 2.5 bl/s water flow velocity groups (*p* < 0.05), and LD concentrations were lowest in the 1 bl/s water flow velocity group, which were significantly lower than in the 2.5 bl/s group (*p* < 0.05), with no significant difference from the control and 2 bl/s groups (*p* > 0.05). There was no significant difference in blood COR concentration of coral trout among the water flow velocity groups (*p* > 0.05) and no significant effect of water velocity on coral trout stress levels (*p* > 0.05).

**Figure 2.** Effect of water flow velocity on blood biochemistry of fish (*Plectropomus leopardus*), (*n* = 6). (**a**) Blood glucose concentration; (**b**) lactic acid concentration; (**c**) cortisol concentration. Different letters indicate significant differences among groups (*p* < 0.05). No letter means no significant difference among groups (*p >* 0.05).

#### *3.3. Intestinal Digestive Enzyme Activity*

Figure 3a shows that the intestinal LPS activity of coral trout was not affected by water flow velocity (*p* > 0.05) and was maintained at normal levels in all groups. Intestinal α-AMS activity was significantly (*p* < 0.05) affected by water flow velocity (Figure 3b), with the values in the control and 1 bl/s water flow velocity groups both significantly higher than in the 2 bl/s and 2.5 bl/s water flow velocity groups (*p* < 0.05), with no significant difference among the control and 1 bl/s water flow velocity groups (*p* > 0.05). Water flow velocities above 2 bl/s reduced the digestive capacity of the fish intestine and affected growth and development.

**Figure 3.** Effect of water flow velocity on the activity of intestinal digestive enzymes in fish (*Plectropomus leopardus*), (*n* = 6). (**a**) Lipase activity; (**b**) α-amylase activity. Different letters indicate significant differences among groups (*p* < 0.05). No letter means no significant difference among groups (*p >* 0.05).

#### *3.4. Intestinal Structure*
