Ammonia is present in the aquatic environment due to agricultural runoff, sewage effluents and decomposition of biological waste. High level of ammonium in the aquatic environment or in aquaculture farms is a serious problem for aquatic organisms. In the present investigation, the sublethal effects of total ammonia nitrogen (TAN) at 9.98 mg L−1 on plasma electrolytes (Na+, K+ and Cl−) level of Indian major carp Catla catla fingerlings was studied for a period of 35 days under semistatic conditions. Exposure of fish to sublethal concentration of TAN at 9.98 mg L−1 caused changes in the levels of plasma electrolytes (Na+, K+ and Cl−) compared with control fish. No mortality was observed in any group during the treatment period. The present study reveals that the levels of plasma sodium and potassium (except on 7th day) electrolytes were significantly (P < 0.05) decreased throughout the study period. However, the decrease in plasma chloride level was found to be not significant when compared with control group. The present study concludes that the measurements of plasma electrolytes levels can be used as potential biomarkers for monitoring nitrogenous chemicals in aquatic environment.