High feed prices and limited access to feed are two critical problems faced by fish producers. One approach that farmers may consider is using less expensive and locally available components. We conducted a 60-day feeding trial to investigate the consequences of the dietary addition of Hemicell® (HC) as exogenous β-mannanase and lavender flower meal (LFM) in low- protein diet (25 % CP) in Oreochromis niloticus. The investigation assessed growth, feed utilization, somatic, body composition and hemato-biochemical indices, and digestive enzyme activity. Fish (N = 180; average weight: 17.17 ± 0.72 g) were haphazardly organized into six groups in triplicates (10 fish/replicate) and stocked in 18 plastic tanks (60 L). For 60-days, fish groups were fed on six prepared experimental diets. The negative control group (NCG) and positive control group (PCG) were fed basal diets containing 30 % CP and 25 % CP, respectively. Groups (G1-G4) were fed on basal diets containing 25 % CP and supplemented with 0.25 g/kg HC, 0.5 g/kg HC, 0.25 g/kg HC+ 10 g/kg LFM, and 0.5 g/kg HC+ 10 g/kg LFM, respectively. The findings showed decreases in the growth, body composition (crude protein and lipids), and white blood cells count (WBCs) in PCG as compared with the control group (NCG); meanwhile, no significant alteration was recorded for other parameters. Dietary addition of HC alone or with LFM enhanced growth, feed utilization, digestive enzymes activity (trypsin, amylase, and lipase), serum total protein, and hematological indices (red blood cells count, WBCs, and Hb) as compared with PCG. The highest results were notable in G2. No marked impacts were observed on water quality parameters, fish survival, somatic index (hepato-somatic and visceral-somatic), and the level of cholesterol, glucose, and creatinine in fish serum by dietary HC or LFM. However, values of triglycerides, urea, hepatic enzymes (alanine and aminotransferases), and body crude lipids increased by supplementation of HC alone or with LFM. Our findings demonstrated that feeding O. niloticus a low-protein diet supplemented with HC (0.5 g/kg) could promote growth, digestive capability, and health status. This provides a platform for potential exogenous enzyme (HC) use in low-protein diets to reduce feed costs and support the aquaculture industry.