Gold-fisher, you may have trouble keeping gold fish. I am assuming that a water temperature of less than 24 would be hard to maintain unless you have a method to cool the water. Anything much warmer than 24 will be too hot for long term health of a goldfish. In addition, the advice from others here that a 5 gallon is too small is correct, I have never seen a goldfish that could be kept very long in a 5 gallon. It might work for a month or two for a very small goldfish but they do not stay small. An adult goldfish will need at least 20 to 30 gallons just for one fish. You don't need to add another 20 gallons for the second one but also don't want to crowd the fish. If you decide to keep tropical fish instead, you will have an advantage over most of us, you can probably keep a tank warm enough for them without a heater or with a very small heater.