A method for increasing the weight and growth rate of fish, preferably Salmonidae, and for preventing and/or treating microbial infections, comprising the steps of a) adding one or more species of probiotic bacteria to water, wherein the species of bacteria comprises or consists of Aliivibrio njordis, Aliivibrio balderis and/or Aliivibrio nannie; and b) exposing the fish to the water containing the one or more species of probiotic bacteria. Preferably, the bacterial strains Aliivibrio njordis strain B1-25, 8-1/2013 mandib V11, Aliivibrio balderis B1-24, 18-1/2013 kidn V12 and/or Aliivibrio nannie B8-24, 313/2013 kidn V13 are used.