We assessed the effect of two sets of water conditions, (i) freshwater (0‰ salinity) at normal ambient temperature (28°C) and, (ii) brackish water with a salinity of 12‰ at an elevated temperature of 34°C, on survival, growth and the utilization of energy, protein and amino acids in fingerlings of red hybrid tilapia (Oreochromis mossambicus x O. niloticus) at feeding rates of 0 (fasting), 25%, 50%, 75%, 85% and 100% (apparent satiation) for 28 days. Fingerlings of uniform weight (13±0.09 g) were acclimated to the water conditions for six days before being assigned randomly to 250 L composite tanks with the designed temperature and salinity, at a density of 36 fingerlings per tank. Fingerlings were fed commercial formulated for tilapia. Fingerling weight and content in the fish body of crude fat, crude protein and amino acids were measured at the start and at the end of the feeding experiment. Survival rates and growth rates increased with increasing feeding rate, but at most feeding rates were marginally lower at 34°C-12‰ than at 28°C-0‰. Energy, crude fat and crude protein in the fish body all increased with increasing feeding rate. The maintenance requirements for energy and for protein were both significantly higher at 34°C-12‰ than at 28°C-0‰, but there was almost no difference in the utilization rate of either energy or protein between water treatments. The maintenance requirements for all sampled amino acids were also higher at 34°C-12‰ than at 28°C-0‰. The utilization rate of most amino acids was slightly higher at 28°C-0‰ than at 34°C-12‰, but it is unclear if these differences between the two treatments are functionally significant. Our results suggest that red hybrid tilapia was cultured in the throughout condition close to their isotonic threshold at a temperature of 34°C and salinity of 12‰, resulting in lower feed intake, lower apparent digestibility coefficients for key amino acids, lower body levels of crude fat, crude protein, and energy, which, in turn, reduce survival rate and growth performance.