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This study aimed to verify whether habitat complexity, assessed through macrophyte biomass, can mediate interactions between fish and invertebrate communities and help to explain the coexistence of congeneric fish species. Fish and invertebrates were sampled in ten macrophyte stands of different biomass in the Upper Parana ' River floodplain. The abundance and richness of invertebrates and the diet and trophic niche breadth of Moenkhausia bonita and Moenkhausia forestii, as well as the diet overlap between these two species, were evaluated. Invertebrate abundance increased with increased macrophyte biomass but richness was not significantly affected. The diet of both fish species differed between low and high biomass stands, with the main difference being a decrease in aquatic invertebrate consumption and an increase in plant consumption with increased macrophyte biomass. In addition, the consumption of items relied on their profitability and accessibility in the environment. The trophic niche breadth of both fish species increased with increased macrophyte biomass, but there was no niche overlap. Our results indicate that even with an abundance of invertebrates, there was a decrease in the consumption of this resource, suggesting that more complex stands can be more effective refuges for aquatic invertebrates. These results highlight the role of habitat complexity in mediating interactions between fish and invertebrates and promoting their coexistence, which also helps to explain the coexistence of congeneric fish species.