The specific water temperature at which fish die varies depending on the species. Some fish, such as trout and salmon, can tolerate cold water temperatures better than others, such as tropical fish. However, even fish that are adapted to cold water can die if the temperature drops too low.
There are a number of things that can be done to protect fish from cold water temperatures. One is to provide them with a heater. Another is to insulate the aquarium or pond. Finally, it is important to monitor the water temperature and make sure that it does not drop too low.