In Northern Europe, small shallow lakes and ponds often becomes anoxic for several months. The only fish that survives in these waters is the crucian carp (Carassius carassius), which is arguably the most anoxia-tolerant fish species. Among vertebrates, its anoxia tolerance is only matched by that of some North American freshwater turtles. However, unlike turtles the crucian carp remains active during anoxia, although at a reduced level. The key adaptation allowing a continued high level of activity is probably its exotic ability to convert lactate to ethanol. Nevertheless, producing ethanol and releasing it to the water is a wasteful strategy as an energy-rich hydrocarbon is forever lost. Indeed, when faced with falling oxygen levels, the crucian carp strives to take up the little oxygen there is in water, thereby avoiding to turn on ethanol production as long as possible when faced with falling oxygen levels. Thus, the crucian carp has a record high hemoglobin oxygen affinity (P50 = 0.8 mmHg at 10°C), and when exposed to hypoxia, it has the remarkable ability to remodel it gills to boosts its capacity for oxygen uptake. In contrast to turtles, which strongly suppress cardiac work in anoxia, the crucian carp maintains all this function at normal levels even after several days in anoxia. This suggests that it needs an active circulatory system for shuttling fuel and removing ethanol. To maintain activity, also the brain has to remain turned on in anoxia. Still, brain ATP levels and ion homeostasis are protected. While channel arrest appears to be important for reducing energy use in anoxic turtle brain, and there is little evidence for reduced neuronal ion permeability in crucian carp. In crucian carp, elevated levels of the inhibitory neurotransmitter GABA are likely to play a major role in reducing the energy use, which still allows a relatively high level of activity and responsiveness.