AbstractThis research aimed to determine the upper limit level of crystalline amino acid (CAAs) inclusion in tambaqui nutrition. Additionally, we investigated the possibility of total replacement of fish meal (FM) by soy protein concentrate (SPC). An FM‐based control diet was formulated to meet the known nutrient requirements of tambaqui, and three additional diets were prepared by replacing FM protein with 250, 500, and 750 g/kg nitrogen of CAA mixture. The fifth diet was formulated to completely replace the FM by SPC. A growth and a digestibility trial were performed in a completely randomized design with five treatments and three and four replicates, respectively. The optimum replacement level of FM protein by CAA was 280 g/kg based on daily weight gain (DWG). The apparent digestibility coefficients of the nutrients were higher in fish fed the SPC diet than the fish fed the FM diet. The whole‐body lipid and adiposity significantly declined with the increase of CAA in tambaqui diets. Protein and amino acid retention of tambaqui linearly declined with the increase of CAA. Blood chemistry metabolites were not affected by CAA inclusion. We concluded that the SPC could replace FM in tambaqui diets without affecting growth and nutrient digestibility. CAAs could replace FM intact protein up to 250 g/kg (or 97 g/kg inclusion level) without showing any adverse effect on growth and metabolism of tambaqui.