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The microbial diseases cause economic losses in aquaculture on global scale and the use of commercial antibiotics for disease treatment produces undesirable side effects. Because many medicinal herbs have evolved potent defense against pathogenic bacteria, there is a growing interest in these herbs as sources for natural antibacterial agents. In this context, green tea (GT), Camellia sinensis L., is considered as a promising source that may be used for improving the growth, survival, and protection of Nile tilapia, Oreochromis niloticus, against Aeromonas hydrophila infection. The present study was undertaken to evaluate the use of GT in practical fish diets and its impact on fish growth and health. GT leaves were ground and added to the ingredients of tested diets to represent 0.0 (control), 0.125, 0.25, 0.50, 1.0, or 2.0 g/kg diet. Fish (1.5–2.0 g) were distributed to various treatments at a rate of 20 fish per 100‐L aquarium and fed one of the experimental diets for 12 wk. After the feeding trial, fish of each treatment were challenged by pathogenic A. hydrophila, and they were kept under observation for 10 d during which abnormal clinical signs and the daily mortality was recorded. The growth‐promoting influences of GT were observed and the optimum growth and feed utilization were obtained at 0.5 g GT/kg diet. The inclusion of GT in fish diet up to 0.5 g/kg diet enhanced the protein contents in fish body, while the lowest lipid contents were obtained at 0.0–0.5 g GT/kg diet. Hematological and biochemical parameters were improved in fish fed 0.25–2.0 g GT/kg diet, while the lowest values were obtained in the control. The survival of fish challenged with A. hydrophila increased with increasing GT level in fish diets. These results indicate that GT supplement is promising immunostimulant, which could improve fish performance, health, and prevent tilapia aermoniosis and the optimum GT level was 0.5 g/kg diet.

The use of antibiotics to control fish disease is one of the constraints in tilapia farming. Hence, the use of probiotic and live organisms as alternative strategies has received much more attention. This study was undertaken to evaluate the use of Spirulina (Arthrospir platensis) as a growth and immunity promoter for Nile tilapia, Oreochromis niloticus (L.). Fish (1.88 g) were randomly distributed at 20 fish per 100 L aquarium and fed a diet containing either 0.0, 1.25, 2.5, 5.0, 7.5 or 10.0 g Spirulina kg−1 diet for 12 weeks. After the feeding trial, fish of each treatment were challenged by pathogenic Aeromonas hydrophila, which was given by an interperitoneal injection, and they were kept under observation for 10 days to record any abnormal clinical signs and the daily mortality rate. The growth‐promoting influence of Spirulina was observed with fish, and the optimum growth and feed utilization were obtained at 5.0 g Spirulina kg−1 diet. No significant changes were observed in fish survival among the different treatments. Spirulina supplementation improved the protein and lipid contents in fish bodies without significant differences among them, especially when fed 2.5–10.0 g kg−1 diet, but no significant differences in the moisture and ash contents were observed among different treatments. The physiological parameters were improved when fish were fed a Spirulina‐enriched diet. However, the highest red blood cells, white blood cells and nitro blue tetrazolium values were obtained at 5.0–10.0 g Spirulina kg−1 diet; meanwhile, the lowest value was obtained in the control. Total fish mortality due to A. hydrophila infection decreased with an increase in the Spirulina level in fish diets. These results indicate that Spirulina supplement is promising as an alternative method to antibiotics for disease prevention in tilapia aquaculture, and the optimum level of Spirulina in the fish diet should be 5.0–10.0 g kg−1 diet.