Afish living in an environment cold enough to freeze the water in its body is not an extremophile.

Well, an extremophile is a microorganism that is capable of living in extreme conditions. Conditions that could be like bone-chilling water or underwater volcanos. A fish living in this cold of environment and is not an extremophile is most likely a native fish to that area. Meaning that this type of fish could have been set in this are by another animal or human. Or, this could be a fish that can adapt easily to its surroundings without needing to be an extremophile. If not any of these, then it is possible the fish will soon die. Hope this helps!