The present invention relates to the use of natural active substances selected from the group consisting of alpha-pinene, alpha-terpineol, cinnamaldehyde, dihydroeugenol, eugenol, meta-cresol and terpinolene in combination with a polypeptide having lysozyme activity in the manufacture of a feed composition for aquatic animals, especially for cold water fish as for example salmon, bream, bass and for warm water fish as for example carp, tilapia, catfish. More particular, this invention relates to the use of a substance as defined above for the improvement of the feed conversion ratio and/or daily weight gain in fish, for regulating the micro flora of the gut and for protecting the animal against infections caused by pathogenic microorganisms, in particular gram+ microorganisms.