Vahed Kiyani - MSc.Student, Filed of Natural Resources-Environment, University of Tehran, Iran
Mohammad Hasan Hosynzadeh - MSc.Student, Faculty Environment & Energy, Islamic Azad University, Tehran, Iran
Mohammad Ebrahimpour - Assistant Professor, Faculty of Agriculture, University of Birjand, Iran

All aquatic organisms are directly or indirectly affected by the physical characteristics of their environment, especially the chemical composition of the water. The high concentration levels, damage tissues and interfere with the normal growth and proliferation. The aim of this research was to Investigation acute toxicity of ZnSO4 and CuCl2 to Gambusia holbrooki by static bioassays at different water hardness. The acute toxicity of selected heavy metals to fish was determined in soft, hard and very hard water, respectively (25, 125 and 350 mg/L as CaCO3). Results showed that water hardness had a significant effect in terms of Cu and Zn toxicity in fish. The toxicity of Cu and Zn increased with decreasing water hardness. Results indicated that an increase in water hardness (from 25 to 350 mg/L as a CaCO3) substantially reduced the toxicity of Cu (up to 38-fold) and Zn (up to 264-fold) to fish at 96 hours exposure. The 96 hours LC50 values for fish were higher in the hard and very hard water compared to the soft water. Water hardness had a much smaller effect on the acute toxicity of Cu than the Zn in fish. Cu toxicity in all water types was higher than Zn toxicity forfish. In polluted areas, it is recommended to expand the aquaculture activities in the water resources that their hardness is higher than the average water hardness in the environment. The toxicity effect of other heavy metals on more diverse aquatic animals should be investigated in the future research.

Acute toxicity, heavy metals, Static bioassay, Water hardness