Water hardness variously influences tissue physiology of freshwater fish - Cyprinus carpio var koi: A report on glucose, oxidative stress and antioxidant biomarkers

Fishes endemic to freshwater habitat are strongly influenced by water hardness with physiological consequences. The present study aimed to understand the effects of four-fold sequential increase from soft to hard waters, on selected tissues of Koi carp, a popular ornamental freshwater fish. Secondary stress markers - Glucose, Oxidative stress (Malondialdehyde - Lipid Peroxidation damage) and Antioxidants (Catalase, Glutathione-S-Transferase and Glutathione) were quantified in gills and white muscle after 14 days of exposure to hardness of 75 (Soft - TS), 150 (Moderate - TM), 225 (Hard - TH) to 300 (Very Hard - TV) mg CaCO3/L. Both the examined tissues were distinctly affected by soft and moderate waters. Glucose in gills (p < 0.05) was proportional to the rise in hardness concentration. Soft, moderate and very hard waters (75, 150 and 300 mg CaCO3/L) affected gills and muscle due to elevated MDA concentrations (p < 0.05). CAT and GST provided antioxidative protection to the tissues. The study results showed tissue-specific differential responses and more importantly, concentrations below 225 mg CaCO3/L elicited strong oxidative impairment in both gill and muscle.