Fish are cold blooded so they rely on the temperature of the water to control their metabolism. Warmer water quicker metabolism and vice-versa. This only works up to a point. Some fish have evolved to operate/survive etc in cool/cold water and others have evolved to live in warm water. Put warm water fish in cold water and they can not survive because their metabolism slows down and eventually stops. If you add cold water fish to warm water their metabolism races and sometimes they can not breath because warm water contains less oxygen than cold water.