Effects of probiotics Bacillus cereus NY5 and Alcaligenes faecalis Y311 used as water additives on the microbiota and immune enzyme activities in three mucosal tissues in Nile tilapia Oreochromis niloticus reared in outdoor tanks

•Tilapia is a popular fish that is farmed and consumed worldwide, but tilapia aquaculture yields are often compromised by bacterial infection.•We tested the effect of two potential probiotic strains, Bacillus cereus NY5 and Alcaligenes faecalis Y311, on tilapia in aquaculture.•The two probiotics, when regularly added to tank water, improved the water quality.•The two probiotics stimulated specific enzyme activities in the fish mucosal surfaces.•The two probiotics increased the relative abundance of beneficial bacteria and decreased that of harmful bacteria on the fish mucosal tissues.