Many freshwater fish species, including carps, salmonids, and catfishes, have been widely introduced outside their native ranges. These introductions can lead to profound ecological consequences. Here, we assessed the environmental impacts of alien freshwater fish, including their influence on abiotic variables, native species, and ecosystem function in the introduced regions. Following the Extended Environmental Impact Classification for Alien Taxa (EEICAT) framework and associated metrics, we analyzed the impact severity and mechanisms of the records related to alien freshwater fish in the InvaPact database, which used large language models to extract and categorize environmental impacts posed by alien species. We have identified over 8000 records, covering hundreds of alien freshwater fish species observed in all continents except for Antarctic. Various negative impacts were posed on native species by alien freshwater fish through a wide range of mechanisms, with predation being the most frequently reported mechanism, followed by herbivory and competition. Moreover, more than half of these species contributed to population decline or even local extinction of at least one native species. In addition, alien freshwater fish also caused alteration of abiotic environment and fundamental ecosystem functions, leading to water quality degradation and modified food webs. Our results demonstrate the necessity of shifting from species-level monitoring toward multi-dimensional assessments that capture the complex impacts posed by alien species, which can contribute to the development of holistic management strategies.