Tissue changes in common carp, Cyprinus carpio weighing 700g following 15 and 30 days of exposure to sub-lethal concentration of cadmium and effect of the ion-exchanging agent zeolite (clinoptilolite) on the reduction of cadmium toxicity were studied grossly and histopathologically. The fish were divided into four groups in 1000 litre fiberglass tanks filled with well water [hardness 302.6 mg CaCo3/l, pH 7, O2 8 mg/L and temperature 15 ± 2ºC] Group 1 and 2 considered as control groups. Group 1 was without any cadmium and zeolite. Group 2 was with 10g/L (10ppt) zeolite and without cadmium, group 3 was exposed to 30 ppb cadmium and group 4 was exposed to 30 ppb (μg/L) cadmium in addition 10ppt zeolite. The tissue samples from: spleen and kidney were collected 15 and 30 days post exposure and processed by histological procedures. The main lesions in cadmium exposed groups were: (1) proliferative glomerulitis and acute tubular necrosis in kidney; (2) lymphoid depletion in spleen. The histopathological changes in cadmium exposed group were more severe than cadmium plus zeolite added group. The lesions in groups after 30 days exposure to cadmium were more severe in compare with the day 15th. The exposure duration was important parameter in severity of the tissue lesions. In order to know the able of zeolite in absorption of cadmium in contaminated water six glass aquaria supplied with well water without fish were used in this experiment. In three of the aquarium, 30ppb cadmium and 10g/L zeolite were added and in another 30ppb cadmium were added considered as control for 30 days. Cadmium level was evaluated in each aquarium two times weekly throughout the experiment using atomic absorption spectrophotometer. According to the obtained results cadmium was significantly reduced in treated water in compare with control.
