Temperature is the main factor affecting the distribution of the sympatric Amazon fishes Paracheirodon axelrodi and Paracheirodon simulans. Both species are associated with flooded areas of the Negro river basin; P. axelrodi inhabits waters that do not exceed 30C, and P. simulans lives at temperatures that can surpass 35C. The present work aimed to describe the biochemical and physiological adjustments to temperature in those species. We determined the thermal tolerance polygon of species acclimated to four temperatures using critical thermal methodology. We also determined the chronic temperature effects by acclimating the two species at 20, 25, 30, and 35C and measured the critical oxygen tension (PO2crit) for both species. Additionally, we evaluated the metabolic rate and the enzymes of energy metabolic pathways (CS, MDH, and LDH). Our results showed a larger thermal tolerance polygon, a higher energetic metabolic rate, and higher enzyme levels for P. simulans acclimated to 20 and 35C compared to P. axelrodi. Paracheirodon simulans also presented a higher hypoxia tolerance, indirectly determined as the PO2cri. Thus, we conclude that the higher metabolic capacity of P. simulans gives this species a better chance to survive at acutely higher temperatures in nature, although it is more vulnerable to chronic exposure.