Refined fish oil refers to the fish internal lipid which is rich in EPA (eicosapentaenoic acid) and DHA (Docosahexenoic acid).
The general fish contains little EPA and DHA, only in cold and deep sea fishes, like anchovy, salmon, sardine, etc, that are all rich in EPA and DHA , but any other animals which are living in land hardly contain EPA and DHA. It is the reason why we use the deep sea fishes to refine the EPA and DHA.