The feed conversion ratio (FCR) or its reciprocal, the feed efficiency ratio (FER) is currently used to estimate the feed requirements in aquaculture. This has a serious disadvantage. The FCR is a function of feeding rate, initial and final mass at fattening, temperature and quality of feed. Therefore the FCR is specific for any combination of these parameters and has to be adjusted whenever a parameter has changed. The feed costs and the fattening time influence the total costs significantly - a precise analysis of the relationships is necessary. For this purpose, the FER is divided into a feed efficiency factor, describing the assimilated percentage of consumed feed and a fish efficiency function, describing the percentage of assimilated food used for growth. The equation for the fish efficiency is derived from a novel growth equation which involves the relationship between assimilation rate and mass growth. The numerical relations are validated using fattening of Bluefin tuna as an example. Similar to the FCR, the fecal loss ratio (FLR) and the nonfecal loss ratio (NLR) are derived. These functions can be used to compute the quantities of waste products while fattening. In a further step, an equation for the economic feed conversion ratio (eFCR) is derived. Aquaculture facilities can also be regarded as ecosystems. It is therefore obvious to describe the eFCR also by means of sequential ecosystem equations. This allows economic optimizations of the production process with respect to the net present value, taking into account temporal changes of particular parameters.