Very often people think of goldfish as cold water fish. They are in fact, subtropical fish that are tolerant of cold water for periods. The optimal water temperature for this species is in the vicinity of 22-24 degrees Celsius. So, if you find your goldfish are not coping so well this winter, you may introduce a heater and bring the water temperature up slowly at about 1-2 degrees Celsius per day until you reach the optimal temperature. If your fish’s health doesn’t improve, consult your fish vet.