| Summary: | A global interest in Silver Barb (<i>Barbonymus gonionotus</i>) cultivation has arisen because of a combination of declining wild stock and a steady increase in demand and market value. The purpose of the current investigation was to evaluate the impact of extreme water temperature changes on growth, hemato-biochemical responses, pepsin enzyme activity and gill histomorphology of <i>B. gonionotus</i>. Four distinct temperatures (22, 26, 30 and 34 °C) were applied in the laboratory (22, 26, 30 and 34 °C) in triplicate glass aquariums (150 L each). At 30 and 22 °C, respectively, the highest and the lowest length and weight gain, specific, daily and relative growth rates were noted. At 30 °C the lowest FCR was recorded (1.42 ± 0.04). The values related to hematology and biochemistry were found to be within normal ranges; nevertheless, there was a notable variation in the parameters when the temperature changed. Hematological results revealed that RBC, HB, PLT, HCT and PMV levels were significantly higher in groups 30 °C with respect to others. It was evident that cold and heat shock stress was present due to the decline in hematocrit levels and rise in WBC values. Regarding the biochemical responses, the level of triglycerides, cholesterol, AP, Ca<sup>+</sup>, HDL, and HDL/LDL ratio increased significantly higher at 30 °C. Glucose, total protein, albumin, Na<sup>+</sup>, K<sup>+</sup>, Cl<sup>−1</sup>, AST, bilirubin, uric acid, and LDL levels were significantly higher at 22 °C compared to other temperatures. There was a higher pepsin activity between 26 and 30 °C while the digestive somatic index (<i>I</i><sub>DS</sub>) was disproportionate. Histological examination demonstrated the well-shaped gill tissues at 26 and 30 °C compared to distinctive pathology in other temperature treatments. As an end note, the results of the experiment indicated that <i>B. gonionotus</i> should be cultured at 26–30 °C to maximize the overall productivity and the health condition of this economically important fish species.
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