The objectives of this study were to develop nutritional strategies on reducing inorganic phosphate supplementation in feed and minimizing phosphorus (P) waste from feedstuffs . Three levels of monocalcium phosphate (MCP) and microbial phytase at 2000 FYT/kg feed were applied to 75.5% plant-based control diets , which are a c ontrol diet (Diet A, 1.8% MCP) , a suboptimal-P diet with phytase (Diet B, 0.9% MCP) , a P- deficient diet with phytase ( Diet C, 0% MCP), and a P- deficient diet (Diet D, 0% MCP). 400 tilapia with an initial weight of 262-268 g were stocked in each of 8 m3 cages in Lake Toba, Indonesia. Fish were fed to apparent satiation twice daily over a 148-day feeding period. At the end of the feeding trial, the survival rate, biomass gain, specific growth rate (SGR), average daily gain (ADG) , and feed conversion ratio (FCR) were examined. The results from the present study indicated that the suboptimal-P diet with phytase ( 0.9% MCP+phytase ) provided good growth performance and feed utilization comparable to the control group (1.8% MCP). I t suggested that the phytase application at 2,000 FYT/kg feed could release sufficient P from dietary phytate to substitute 0.9% of MCP in Diet B . However, P release from phytase was insufficient in fish fed diet C when MCP was totally removed. The growth and feed performance of fish fed diet D without inorganic P were the lowest among the treatments. It was proven that microbial phytase supplementation at 2,000 FYT /kg feed could improv e P digestibility and utilization in plant-based diets to meet the fish requirement and improve growth performance . Thus, this could translate to the formula cost reduc tion from inorganic P substitution in the formulas and reducing P discharge from indigested phytate P from plant-based diets into the environment.