This study aimed to evaluate how simultaneous fluctuations in temperature, salinity, and electrical conductivity, conditions frequently encountered in North African freshwater systems, affect the growth performance of Nile tilapia (Oreochromis niloticus). Fry were reared for 75 days under two treatments: a control group maintained at optimal conditions (28 °C; 0‰ salinity), and a stressed group exposed to realistic environmental fluctuations, with temperature ranging from 17.64 to 23.64 °C, salinity from 5.03 to 5.42 ppm, and conductivity from 8.99 to 9.65 mS/cm. Growth was monitored twice weekly, and statistical analyses included two-way ANOVA and principal component analysis (PCA). Stressed fish exhibited markedly reduced growth, reaching final values of 7.97 ± 0.00 g and 7.60 ± 0.00 cm compared with 26.29 ± 0.00 g and 11.03 ± 0.00 cm in the control group. ANOVA revealed significant effects of treatment, exposure duration, and their interactions (p < 0.05), while PCA explained 95.7% of the variance and identified body weight as the most sensitive trait to abiotic instability. Although the study was conducted under controlled laboratory conditions and limited to a single salinity level, the results provide quantitative evidence that combined thermal–osmotic instability around 20 °C and 5‰ salinity severely impairs tilapia somatic growth. These findings offer practical reference thresholds for aquaculture operations, particularly in semi-arid regions where environmental fluctuations are common, and highlight the importance of monitoring water mineralization and temperature to minimize chronic stress. The originality of this work lies in its integrated assessment of combined environmental stressors, rather than isolated factors, and in demonstrating the discriminant value of body weight as a biomarker of environmental instability in freshwater tilapia aquaculture.