In view of the diversification of fish farming and new possible candidates for aquaculture, a general approach is needed to define energy and protein requirements so that each fish species can realize its full growth and economic potential. Growth is usually defined as deposition of new body components, which in fish consists mainly of protein, lipid and water. In addition to the requirement for maintenance, the feed must supply the precursors for synthesis of new tissue, and also the energy needed for synthesis of both protein and lipid. It is not always clear if differences in feed conversion efficiencies that are reported for various fish species are due to true biological differences or the different conditions used in the various studies.