Abstract: Laboratory tests were conducted on three fish species viz. Catla catla, Labeo rohita and Cirrhina mrigala to determine their sensitivity to iron and nickel for 96-hr LC50 and lethal toxicity. These tests were performed with three age groups of each fish species at water temperature of 30°C with pH 7 and total hardness of 100 mg L-1. The 96-hr LC50 concentrations of both iron and nickel varied significantly among the three fish species with age. 30-day fish were more sensitive to metals toxicity, followed by that of 60 and 90 day fish, respectively. Among the three fish species, Catla catla were more sensitive to nickel while Cirrhina mrigala showed highest sensitivity to iron. Lethal iron concentrations for all the three fish species ranged between 72.20 mg L-1 (for 30 day Labeo rohita) and 164.65 mg L-1 (for 90-day Catla catla). However, nickel concentrations fluctuated between 20.20 and 50.11 mg L-1 for 30-day Catla catla and 60 day Cirrhina mrigala, respectively. The 96-hr LC50 nickel concentrations in three fish species varied significantly within age groups. However, the difference between 60 and 90 day old fish was statistically non-significant. Among the three fish species, Catla catla appeared significantly more sensitive than that of Labeo rohita and Cirrhina mrigala to nickel contaminations. The 96-h LC50 concentrations of both metals for all the three fish species were positively and significantly dependent upon fish age. The regressions of water temperature on 96-hr LC50 concentrations of both the metals were negatively significant. However, water pH, hardness, ammonia, sodium and potassium showed significantly direct relationships with 96-hr LC50 concentrations of fish. The ammonia concentrations of test medium increased while dissolved oxygen decreased at high levels of toxicants. At higher 96-hr LC50 values of iron, dissolved oxygen decreased significantly in the test medium.