Water runoff encompassing agricultural, urban storm water, and forestry-related overland flow which represents one of the most pervasive non-point source pollutants threatening freshwater and coastal aquatic ecosystems globally. This comprehensive review synthesizes current evidence on the mechanisms through which various categories of water runoff impair fish biodiversity, examining sediment loading, nutrient enrichment, chemical contamination, and hydrological alteration as primary pathways of impact. Drawing on peer-reviewed literature published predominantly in recent times, this review documents how excess nutrients transported by runoff drive eutrophication and hypoxia, how suspended sediments impair fish sensory systems and reproductive success, and how heavy metals and pesticide residues elicit acute and sub lethal toxicological responses across trophic levels. The synergistic effects of multiple stressor, increasingly compounded by climate change, are found to accelerate community restructuring toward pollution-tolerant assemblages at the expense of sensitive endemic species. The review further evaluates landscape-scale and reach-level mitigation strategies, including riparian buffer restoration, constructed wetlands, low-impact development infrastructure, and integrated watershed management frameworks. The paper concludes by recommending critical knowledge gaps and research priorities essential for reversing the global decline of freshwater fish biodiversity in the hyper anthropogenic era.