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Ammonium sulfate [(NH4)2SO4] is one of the most widely used nitrogen-based fertilizer in agriculture, and has been produced for over 150 years. However, limited research has been done to investigate the eco-toxic effects of ammonium sulfate, commonly present in surface runoff. This study therefore aimed to investigate the effects of varying ammonium sulfate concentrations on the normal physiology of Daphnia magna through a modified acute toxicity testing. Concentrations of ammonium sulfate solutions at 0M, 0.05M, 0.10M, 0.15M, 0.20M, 0.25M, and 0.30M were prepared and tested on 10 D. magna for each concentration of ammonium sulfate solution. The bioassay test was done by observing the effects of different concentrations of ammonium sulfate solution on the heart rate of D. magna. The percentage increase in average heart rate of D. magna after exposure to the respective concentrations of ammonium sulfate solution were calculated and a relationship between varying concentrations of ammonium sulfate concentration and the heart rate of D. magna was illustrated by plotting a graph using the respective data points obtained. Results indicated that increasing concentrations of ammonium sulfate solution resulted in an increase in the heart rate of D. magna per minute, up till 0.20M concentration. Increasing concentrations of ammonium sulfate solution beyond 0.20M resulted in a decrease in the heart rate of D. magna per minute. It was also discovered that specifically, the ammonium ions present when ammonium sulfate dissociates in water, is responsibility for toxicity, and not the sulfate ions. It is reasonable to conclude that ammonium sulfate poses significant eco-toxic effects as D. magna is a common primary consumer in many freshwater aquatic ecosystems, any change in its population quality or quantity can cause irreparable effects to the populations of other aquatic organisms.