The study focuses on the use of phytase in aquaculture, its zootechnical benefits and the possibilities of incorporating it into feed. The aim is to reduce phosphorus waste in fish feed, with no loss of zootechnical performance and lower feed costs. We studied the data in the bibliography, in order to better valorize raw materials (total phosphorus, phytic phosphorus and available phosphorus); to determine phosphorus requirements for aquaculture species; to determine signs of phosphorus deficiency in fish; to study antagonism between phosphorus and calcium and also to study the different forms of discharge by rainbow trout. The results found in the bibliography enable us to test several hypotheses for the formulation of feeds for rainbow trout with different raw materials. This simulation and waste calculation enabled us to validate two feeds: a control feed A1 (0.5% monocalcium phosphate) and a test feed A2 (supplementation with 0.002% phytase and phosphate). The feeds were produced and sent to an experimental facility (Lycée agricole de Bréhoulou). The results of the formulation showed a 28% reduction in phosphorus loss for the test feed compared with control feed. Supplementation resulted in a gain of 2.3 euros per tonne. The results show no significant difference in zootechnical parameters (growth rate, mortality, weight gain and obvious conversion) between the control and trial feeds. Waste measurements also show no significant difference between the control feed and the test feed, but the difference would enable waste to be reduced by 35.6% through the use of phytase.