First off, goldfish are coldwater fish. This means that under most household conditions (someone please find an accurate figure) they will be fine without a heater.
Second, if they are two fancy goldfish, I'm willing to bet they are gonna need a bigger tank. I have heard that an individual goldfish needs at minimum 30 gallons.
Third, your title indicates that youmay be considering adding "tropical" fish to your aquarium. In addition to this being possibly overstocked, you also need to remember that coldwater fish and tropical fish do not mix.
Fourth, you have not mentioned your filtration in this tank. This is a vitally important priority, as goldfish are known for their large amounts of bioload.
Fifth, how long has the tank been running and what is your maintanance?
Good luck, Black wolf.