Trait-based approaches, which focus on using species’ traits to explain ecological phenomena, are being used increasingly by fish ecologists to investigate linkages between species life history, assemblage structure and ecosystem functioning. Trait-based approaches have also been applied to biogeography, conservation and management, revealing new insights on dispersal, invasiveness, and extinction risk, to name a few. Researchers working with marine and freshwater fishes have distinct approaches and perspectives toward trait-based studies, but also have much to learn from each other. Here, we summarize those studies, identifying major similarities and gaps. We identify the major ways in which fish traits have been used as an ecological currency, the type of aquatic environment in which they were conducted (freshwater, marine, estuarine) and the traits they used. Traits are grouped into major categories: morphological, ecological, behavioural and life-history traits. Despite similar approaches being applied across different aquatic environments, our analysis reveals major differences between marine and freshwater studies related to the types of traits they used. Ecological and behavioural traits dominate in marine studies, whilst morphological and life-history traits prevail in freshwater studies. We believe that these differences are caused, in part, by variability in trait availability, research practices, and disciplinary norms. Traits of marine fishes are mostly derived from underwater observations using scuba. On the other hand, freshwater studies often involve fish collecting and direct measurement or estimation of trait values. These differences are unfortunate because it hampers the integration needed for advancing research, yet are also opportunities for broaden perspectives in the future. Our aim is to promote the integration between these two ‘cultures’ by showing how some traits used in freshwater studies can greatly improve marine studies and vice-versa. Finally, we identify information gaps and propose solutions to broadening the scope of traits used to answer ecological questions.