From 45 private fish farms, 641 tilapia samples were collected, which suffered from tilapia summer mortality outbreaks at different localities. The findings of the conventional test and 16S rRNA gene sequence recognition indicated that the isolated bacterium from the examined farms is Aeromonas veronii. A 15-day feeding trial was conducted to determine the effects of formic acid (FA) on biochemical parameters, immunological status, cortisol levels and total gut bacterial population of A. veronii-challenged tilapia. Fish were divided into five groups. The first and second groups served as control-negative and control-positive groups, which were intraperitoneally injected with 0.2-ml sterilized tryptic soy broth and A. veronii (4.3 × 106), respectively, and both were fed with a basal diet. Groups 3–5 were intraperitoneally injected with 0.2-ml A. veronii (4.3 × 106), fed a basal diet until clinical signs appeared and fed on a medicated diet containing florfenicol 15 mg/kg fish body weight and 0.5% and 1% FA respectively. The results suggest that including 1% FA in the diet lowered mortality and relieved the pathogenic effects of A. veronii on the general health status of tilapia fish.