The Amazon basin houses a particular group of freshwater organisms, whose study tells the geological history of the region, how biological diversity was shaped, how it is maintained, and what it hides. The fish of the Amazon is represented by more than 3,000 known species distributed from the basal groups up to the more specialized ones. This species richness hides a diversity of adaptations that are dispersed at all levels of the biological organization. In this summary of the conference delivered at the Brazil-France Bilateral Symposium, held in Manaus in 2018, we describe two aspects of the hidden world of adaptive diversity: adaptations to changes in dissolved oxygen and the abilities of the Rio Negro fish to face acidic and ion-poor waters of their habitats. Also, we present the vulnerability of Amazonian fish to ongoing climate changes. Then, very briefly, we call attention to the many hidden biological processes that allow these fish species to survive their environments, much of which is unknown. Indeed, however, they are intricately related to men, either by responding to environmental disturbances that we have caused, or by containing information that contributes to improving the quality of the environment in which we live.