Personally I would get a two more goldfish and get him a 55 gallon. The fish would love you for that. I think 55 is about bare minimum for a 14" fish. Anyways IME goldfish tend to do better in groups. Also don't bother with a heater. Your goldfish will be just fine. I have goldfish in an outdoor pond in michigan. And I haven't really seen any 14" goldfish. I know that some get that big but I think it is more common to see about 8-10".
Chad