The present trial was conducted to study the effects of C/N ratio to biofloc technology (BFT) at different salinities on water quality, growth performance, and economic feasibility of flathead grey mullet fingerling. BFT was in natural light (12:12h light: dark schedule) with concrete tanks (size 6m3 water), filled with underground marine and tab water, representing three salinities (fresh, brackish 15.5ppt, and saline 33ppt). Diets contain crude protein (20%, 24%) were applied under the biofloc system, two levels of C/N ratio through adding 60% starch of the produced daily diet. Two biofloc treatments under different salinity were managed. BFT ponds were aerated and agitated using an air blower. Forty-two fingerlings with an initial body weight of 10.89 ±0.12 g/fish of grey mullet were stocked in each pond. Feed was applied daily at 3% of the total fish biomass in each pond. Survival rates of the mullet were above 91%, with a significant (P≤0.05) difference for the brackish water with both (20%, 24% CP). Regarding growth performances of the grey mullet at the three salinities with 60% starch, the highest final body weight (FBW), weight gain (WG), and specific growth rate (SGR) values were recorded for fish reared in fresh and brackish water, respectively with no significant differences due to salinities or diets. Water quality parameters alternated between safe values except for nitrate, and no significant (P>0.05) difference was recorded for zooplankton counts. No significant effects of salinity on the mullet growth were recorded under BFT.