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Behavior is a mechanism through which organisms react to internal and external stimuli to best cope with challenges in an ever-changing environment. The study of animal behavior patterns in response to environmental stress/threats, is a relatively new and unexplored topic. The aim of this study is to offer a modest contribution in explaining the effects of pharmaceutical pollutants found in freshwater ecosystems, using the behavior patterns and physiology of Carassius auratus. Behavior changes were evaluated through swimming patterns, opercula and pectoral response, and rheological aggressivity. Animals were exposed for 5 weeks to water (as control), ethanol (EtOH, 0.25 and 1%, v/v), fluoxetine (FLX, 100 μg/L) and caffeine (CAF, 50 mg/L) and their short-term responses were recorded. The video has been analyzed using the open-sourced software program Track3D and EthoVision XT, which objectively quantified swimming and social behaviors. In all treatments, fish showed significantly (P<0.01) high level of stress, aggressivity and hyperactivity, compared to control. An interesting fact was that for each pollutant, fish exhibited different swimming patterns, from the normal one. These changes in the nervous system such as stressed behavior, irregular swimming patterns, hyperactivity and aggression, are consequences of pharmaceutical pollution in freshwater bodies and as such they can be used as suitable early physiological response biomarkers to environmental stress. Monitoring of altered behavior is a great early indicator of water pollution, which can easily be applied in the best aquaculture and fishery practices.