i kept 4 comets and one oranda in a tropical tank 26-28 degrees c for two years, in that time they grew from 4cm to 20cm, spawned several times, ate 20 fully grown penguin tetras, and got so big and fat that i ended up giving them to ronnie to go in his auckland outdoor pond. where they are to this day, still growing, still eating like pigs...
considering goldfish originally come from tropical climates, and are kept all over the world in places like america where fish tanks sit at tropical temps without heating, it is only logical to assume that they will be fine in tropical temp waters. logic aside, i, and many others have seen first and second hand how successful goldfish are in tropical temps. in fact, i believe that keeping them in an unheated tanks or ponds over NZ winter where temps can drop to freezing is a lot worse than keeping them just a few degrees above the "norm". for the fishes comfort, it's most important to keep the temperature constant and consistent.
i apologise on behalf of fnzas that you are getting conflicting advise here. do some research on the internet and you will find the truth for yourself, there is tons of information out there on keeping gold fish.