Hi Laura,

the best temp for goldfish is 2o to 25 C, main thing is the temp is constant.
The lower the temp is the less algae grow.
My goldfish do have an average temp of 28C, this is because we live in the sub tropics and is is average day temp now in autumn. They are happy, do not show any discomfort.
We got our fish 4 weeks ago, for summer we will have a chiller.

Hope this helps

Temperature is actually just as controversial as feeding when it comes to goldfish. And even though they enjoy 19-20 degrees Celsius, they live perfectly in 21-25 degree water as well (: They can easily survive even lower temperatures outside in ponds in the winter, as they are highly adaptable fish!
Although, goldfish who live in about 26 degrees year-round will grow more, have better coloration and be healthier! And the risk of swim bladder disorders is lower (: Besides, a healthy carpet of algae on the tank sides will only give protection and a nice snack to your fish (: At the same time as it consumes wastes. Green algae is also a sign of good water quality.
So really, don't be afraid of algae!

Or you could simply put a bit colder water in your tank during water changes! About 1-2 degrees shouldn't hurt your fish.
On the contrary I read that it's pretty good to add colder water into the aquarium sometimes because it makes the fish more active (:

Hahhahahahaha, yes this is so true, the water I add is always cooler, not much though, yes about 2-3 C less than the water in the tank, and hahahaha, they are so active , seem to turn into race horses, LOL

The fresh salted water really does them good