Social media platforms have increasingly played an influential role in boosting the online wildlife trade, by providing an easily accessible way to share information, advertise specimens, and arrange sales. We investigate how the online trade in freshwater and marine fishes for ornamental purposes operates through social media in Brazil, aiming to gain an understanding of the characteristics of that trade, and of the actors operating in it. A six-month monitoring of advertisements posted on Facebook groups was conducted, through which we recorded 1121 posts announcing the sale of ornamental fishes, totaling 5005 specimens belonging to at least 608 species, 66% of which were non-native to Brazil and 25% forbidden to be traded by national legislation. For freshwater fishes, native species were the most advertised and at a lower number of specimens/ad than marine fishes, which, conversely, mostly comprised non-native species. We found that higher prices were associated with non-native, prohibited, and large-sized species. Brazil is both an important commercial market for ornamental fishes, and one of the top countries for the use of social networks. Therefore, it may lend itself as a useful case-study that can help increase our general understanding of the functioning of the online wildlife trade. The high number of ads and species involved recorded, coupled with the potential offered by the online trade to increase unmonitored and unregulated sales are significant issues that need to be addressed by national environmental agencies and social media platforms.