Disclosed herein are novel microbes Thraustochytrium sp. KJS-1 and Bacillus polyfermenticus KJS-2. Also provided is an aquaculture feed additive for use in fisheries, which comprises freeze dried Thraustochytrium sp. KJS-1, freeze dried Bacillus polyfermenticus KJS- 2, and freeze dried Bacillus licheniformis. In addition to providing DHA, an essential nutrient for fish, the aquaculture feed additive shows antibacterial and antifungal activity, thereby preventing fish from dying from bacterial and fungal diseases.