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Abstract In tropical and highly diverse systems, Seed Dispersal Effectiveness (SDE) is a very useful approach to investigating the role of seed dispersers in one of the most important and complex phases of plants by analysing the quantitative (QC) and qualitative (QL) components of the seed dispersal process. Although fishes are recognized as an important guild of frugivores in flooded environments, we still know very little about the dispersal effectiveness performed by this group. Our study evaluated two quantitative subcomponents of effectiveness, where we used: (a) the volume of intact and crushed seeds found in the stomach and intestine of fish (QC 1 ), and (b) the total number of tested and viable seeds (QC 2 ). From these subcomponents, a two‐dimensional landscape of effectiveness was generated and a fish species was ranked according to the quantitative component of the SDE. Eight species of fish consumed 1054 seeds belonging to 16 plant species. Sphinctanthus striiflorus , Corynostylis pubescens , Connarus sp. and Piper sp., were the most abundant plant species in fish's diets, totalling 95% ( N = 983) of the seeds consumed. We used a tetrazolium viability test protocol and about 678 (85%) of the 797 seeds tested were viable and 119 (14.7%) were unviable; the viability test was inconclusive for less than 1% of the seeds. Our findings showed that, as seen in other effectiveness studies with terrestrial vertebrate dispersers, fish species also have different degrees of effectiveness. Additionally, we pointed out that the fish species more specialized in the consumption of fruits and seeds were quantitatively more effective dispersers, while species with lower SDE corresponded to generalists that just occasionally consume seeds. We suggest that high‐diversity fish assemblages allow for greater complementarity in the seed dispersal interaction, which can potentially favour the recruitment of plant species via ichthyochory.