Formulas for balanced fish diets must include an energy source plus sufficient indispensable amino acids, essential fatty acids, and specific vitamins and minerals to support life and to promote adequate growth. Tentative requirements for the 10 indispensable amino acids and evaluation of the sparing effect of several others have been reported for only a few species of fish held under specific experimental conditions. These findings must be confirmed for other species and other rearing conditions, since the protein requirement and amino acid balance is the major determinant for growth, and is the greatest food source problem. Essential fatty acid needs for fish reared indicate that this component may be the second limiting factor for future practical diets. Water-soluble and fat-soluble vitamin needs have been reported for only a few fish and these data must be extended to include other species and fish reared in different waters. Digestion coefficients of diet ingredients, measured for terrestrial animals, must be confirmed in fish before data banks of nutrient ingredient sources can be translated into least-cost computer diet formulas. Specific nutrient requirements vary with species, size, water temperature, stress, and nutrient balance; thus, economical rations must be programmed accordingly. Techniques that have been developed for determining metabolizable energy and digestibility of diet components for several species can be expanded to develop diets and rations for scientific fish husbandry.