This study was conducted to evaluate the effect of using protease in diets of Nile tilapia on growth performance, water quality, blood parameters and intestinal morphology. The cost of these diets and their return on fish performance was calculated. A total of 360 fish were randomly allocated into four groups with triplicates (30 fish per replicate). Four diets were formulated; two controls (without protease supplementation) and two experimental diets (supplemented with protease). The first control diet contained the normal protein requirement (30% CP; control +ve), while the second control had a low protein content (29% CP; control −ve). The third diet was supplemented with protease at a dose of 500 g/ton, and its CP content was reduced to 29.0%, by reducing the fish meal content. The fourth diet contained the same CP level as the first control (30%) and supplemented with 250 g protease per ton feed. The experiment lasted for 14 weeks. The results showed that body weight and length, weight gain, specific growth rate, feed intake and feed conversion efficiency in the control −ve group (low CP) supplemented with protease were similar (p > 0.05) to that of the control +ve with normal CP content. However, these performance parameters were lower (p < 0.05) in fish fed low CP diet without protease supplementation. Providing protease to the control +ve diet improved all measured performance indices. The ammonia and nitrite concentrations of the water were reduced (p < 0.05) in control −ve and protease‐supplemented groups. The height and width of intestinal villi were increased (p < 0.05) in fish fed diets containing protease. The inclusion of protease reduced the diet cost and also the feed cost of fish weight gain. In conclusion, supplementation of protease can improve the productive performance of fish, spare dietary protein and produce economical diets. Moreover, it can help in improving the water quality of fish via lowering the ammonia and nitrite contents, or through increasing the degradation of dietary protein.

This study was conducted to evaluate the effects of Spirulina platensis in Nile tilapia diets on growth performance, blood hematological and biochemical parameters, immunological status, and intestinal histomorphometry. A total of 228 fish were randomly allocated into four groups with triplicates (19 fish per replicate). The first group was fed the control diet, which contained no Spirulina supplementation. The other three groups were fed diets containing graded levels of powdered Spirulina: 2.5%, 5.0%, and 10.0% in the second, third, and fourth group, respectively. S. platensis was added to the diets partially substituting the fish meal content. The experiment lasted for 8 weeks. The results showed that dietary Spirulina supplementation improved (P < 0.05) the body weight and length, weight gain, specific growth rate, condition factor, and feed conversion efficiency. Moreover, Spirulina increased significantly (P < 0.05) the hemoglobin, PCV, RBCs, and WBCs count. Also, it increased the lymphocytes, eosinophils, IgM level, lysozyme activity, and phagocytic activity in the blood. Additionally, the Spirulina raised (P < 0.05) the serum albumin level but reduced (P < 0.05) the creatinine and urea levels. The addition of Spirulina increased (P < 0.05) the height and width of intestinal villi and the lymphocytes and goblet cells count in the intestine. The obtained results were increased by increasing the inclusion level of Spirulina, especially for body weight and length, weight gain, FCR, phagocytic activity, and intestinal parameters. In conclusion, supplementing S. platensis can improve the growth performance of fish. Moreover, it can stimulate the immunity of fish through increasing the level of immunological blood indicators (IgM, lysozyme, phagocytic activity, lymphocytes, and eosinophils) as well as the local intestinal immunity (lymphocytes and goblet cells). So, it can be recommended to use S. platensis in fish diets not only to improve the growth performance but also to enhance the immune status.