The aim of this study is to determine the effect of (1) different pH levels (5, 6, 7, 8, and 9) with water hardness 20 mg/L CaCO3 and (2) pH 5 and 7 with varying water hardnesses (20, 60, and 120 mg/L CaCO3) on the intensity of infection by Ichthyophthirius multifiliis in fingerlings of the silver catfish, Rhamdia quelen. In Experiment 1, the lowest and highest mortality rates were observed in fingerlings exposed to pH 5 and 9, respectively; in Experiment 2, fish kept at pH 5 and 7 with hardness 20 mg/L CaCO3 showed significantly lowest and highest cumulative mortality, respectively, than those maintained in other conditions. The lowest intensity of trophonts/fingerling was observed at pH 5 and hardness 20 mg/L CaCO3 in both experiments. We conclude that infection by I. multifiliis is less severe in silver catfish maintained at pH 5 and hardness 20 mg/L CaCO3. High water hardness led to an increased intensity of trophonts and decreased survival in silver catfish kept at pH 5, but decreased intensity of trophonts and improved survival when the fish were kept at pH 7. © by the World Aquaculture Society 2011.
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