From another point of view, KDF can improve the general health status of cultured animals by its stronger antimicrobial effect towards coliform bacteria, Escherichia coli and Salmonella sp., than towards lactobacilli [3]. It was reported that the total bacteria per gram of faeces was significantly reduced in the fish fed with an organic acid blend and KDF diets [10]. Similarly, Da Saliva et al. [13] indicated that propionate, butyrate and acetate salts exhibit the highest inhibitory capacity against vibrio species in marine shrimp. These acids can penetrate through the cell wall of gram-negative bacteria and release protons into the cytoplasm. Thus, the bacteria consume a large amount of ATP to excrete protons in trying to maintain a balanced intracellular pH, resulting in the depletion of cellular energy with eventual cell death [14]. Although the scientific publications focused on the antimicrobial effects of organic acids are numerous, very few publications have tackled their effects on the indigenous beneficial flora, lactic acid bacteria (LAB), which has become a major source of concern as one of the most common probiotic bacteria used in aquafeeds [15]. To our knowledge, there have been no previous reports about the ability of acidifiers to influence the humoral and cellular non-specific immunity of cultured tilapia. As a result, the current study was planned to assess the effect of potassium diformate, KDF (Aquaform)® on the growth performance, protein digestibility, gastrointestinal pH, gut beneficial flora, innate immunity and survival of Oreochromis niloticus challenged with pathogenic Aeromonas hydrophila.