Their secret is anaerobic respiration - releasing energy without needing oxygen. Though it's 15 times less efficient than aerobic respiration (with oxygen), which is what animals normally do, by slowing their metabolism, crucian carp can survive up to 140 days in anoxic ponds and lakes that have frozen over. (Goldfish can do it too, but can't survive early as long.) By staying in isolated ponds, they're safe from predators, and the low temperatures lower the metabolic rate.
|From Hakon, Flickr|
To keep functioning properly, crucian carp have to get rid of the products of anaerobic respiration, like lactate, which would otherwise build up. The enzyme alcohol dehydrogenase (ADH) is unique to crucian carp and goldfish, and it converts the waste products into ethanol, which readily diffuses through the carp's biological membranes, into the blood and out of the system through the gills.