Unlike endotherms which can metabolically control their own body temperature, ectotherms rely upon environmental temperatures for thermoregulation. Most fish are ectotherms. Ectothermy can be metabolically more efficient than endothermy because organisms do not have to expend any energy to self-regulate their body temperatures. However, they are at the mercy of their environment more than endotherms because they use ambient water temperature to control their body temperature.
Poikilothermic fish have no control over their body temperature whatsoever. Their core body temperature conforms entirely to ambient temperature and can fluctuate widely. This means that their organs and enzymes need to be capable of functioning at a range of temperatures. As physiological processes have often evolved to operate most efficiently at certain temperatures, ectothermic fish have evolved multiple strategies to maintain optimal thermal habitat. Stenotherms live within narrow environmental temperatures in contrast with eurytherms which can live in a wide range of environmental temperatures.