The Seydisuyu River one of the tributaries of Sakarya River which is the third largest river system in Turkey. The Seydisuyu Basin includes one of the most important boron mines in world. Boron sources in the Seydisuyu Basin, fertilizers, and pesticides used in agricultural applications; geological structure of the basin and mining applications are resources important for boron and arsenic. Kunduzlar Dam Lake, which is one of the most important reservoirs of the Seydisuyu basin, was constructed for the Seyitgazi Plain in 1983. Boron and arsenic values were the two important contaminants determined around the region. B and As levels were determined in water and sediment of Kunduzlar Dam Lake and also B levels were identified in tissues of Carassius Gibelio. Water and sediment samples were collected seasonally between 2011-12. Also, muscle, gill, and liver tissues of Carassius Gibelio (Bloch, 1782) were examined in spring 2012. Arsenic and boron levels in water were found higher than permissible levels. In all tissues of Carassius Gibelio bioaccumulation factors (BAFs) for boron were remarkably high and boron accumulation levels in the muscle were lower than other tissues.