Goldfish do not like tropical temperatures. However, the smaller varities of fancy goldfish have been bred to be indoor pets for over 1,000 years now. They are a uniquely room temperature fish. If your tank remains about room temperature, 72 degrees farenheit or 22 degrees celsius, all year round then your fish will be fine. However, if you live somewhere in which it snows for a good portion of the year then, you need to invest in a thermostatic heater.
A thermostatic heater is a submersible heater designed for aquariums that can be set to a specific temperature of your choice. You should set your aquarium heater to 72 degrees farenheit or 22 degrees celsius.
Other advantages to adding a heater to your goldfish aquarium include, avoiding frequent temperture changes that stress your fish, and increasing the growth rate of your fish. Keeping your fancy goldfish in water above 68 degrees farenheit or 20 degrees celsius will boost the metabolism of the fish. Essentially, the fish will be more active, have a better appitite, and reach a larger size faster than other goldfish.