The ideal temperature for goldfish is around 22.2 degree Celsius or 72 Fahrenheit.  Although some people will say you don't need a heater in a goldfish tank, there are exemptions though.  Most home aquariums are relatively small and the water temperature can drop very fast.  Rapid changes in temperature can be very stressful for them and weaken their immune system, making them susceptible to a lot of different diseases. All fish in general are cold blooded and thus are at the mercy of their of their environment. Why subject your pet to such stressful conditions when this can be totally avoided. A heater is a relatively cheap investment, compared to the costs of medicines you would have to buy to treat the fish if it becomes sick as result of the having to deal with all the temperature changes. I would buy one and set it to 21'C - 22'C and you won't ever have to worry about the temperature of the room in the winter again.  I don't recommend allowing water temperature for fancy goldfish to ever drop below
15'C  .

Hope this helps

Ah ok. Thankyou My room gets quite cold in winter as it only gets afternoon sun so after hearing this I will buy a heater.

Yeah, good question Because my water can get that warm in summer too.

Hanna: At the moment there is a huge difference in night and day temps, the days are like nice summer days and the nights are like winter nights should be. This is highly unusual. However, my room is spared from this because it stays quite cool most of the time. So Sunshine isn't exposed to extreme changes, it just might drop a bit on a very cold night.