Dissolved oxygen is a key environmental parameter for aquatic life. In two sets of experiments we evaluated the effect of long-term changes in the dissolved oxygen environment on the morpho-physiology and behaviour of the weakly electric fish Petrocephalus degeni. First, we captured fish from a severely hypoxic habitat in Uganda and acclimated them to normoxic conditions for up to 73 days while measuring respirometric and morpho-physiological traits in the field. In the second experiment, we used fish that had been kept under normoxic conditions for over two years in the laboratory. We assessed their hypoxia avoidance behaviour as well as physiological traits before and after 56 days of exposure to hypoxic conditions (15% air saturation). We found that gill size changed significantly in both experiments. Blood haemoglobin concentration was only affected during normoxia acclimation. Other traits, such as critical oxygen tension and hypoxia avoidance behaviour, were unaffected by the acclimation treatment. Our results suggest that P. degeni remain remarkably hypoxia tolerant throughout acclimation to different oxygen environments. Plasticity in respiratory traits that are directly associated with oxygen extraction and carrying capacity likely facilitates the homeostasis of higher-level traits linked to hypoxia tolerance, such as critical oxygen tension and hypoxia avoidance, by optimizing energetic trade-offs in different dissolved oxygen environments.