We’re considering starting up a 10 gal tank with goldfish. The room is quite chilli. (It’ll be winter here…) So, I wonder if I will need a heater? What temp range do goldfish prefer? …As I kid, I never gave it a thought and the carnival-won fish survived just fine.
What would Dennis say, Stanley? (ref: “Stanley” cartoon)
Generally speaking, if you’re comfortable, your goldfish will be comfortable, too (unless you’re one of these “overheated furnace air is bad for you!” health fanatics who keeps the thermostat at 62). 68 to 72 F is good for goldfish. So no, generally they don’t need a heater.
I sure hope my goldfish likes chili! Sorry for the typo in the OP; I’m fatigued, exhausted, spent, pooped! Anyhoo, if the room is colder than 68 F, it’d be a no-no, huh? It’s possible the room may be as cold as 60 F…we’re in a cut-back mode ($$$) to stop supporting the local utilities!
…Ever seen a goldfish knit a sweater?
Standard Goldfish are essentially carp. The temperature you keep them at only affects how active they are. I have a garden pond that I stock with cheap pet store Goldfish. The start to slow down when the water temperature drops below 50F. Below 40F they kind of drift down to the deep part of the pool where they “hibernate” until spring. As long I keep a small pump going so the ice doesn’t freeze over solid, they “wake up” in the spring with no problem.
A heater for Goldfish would be a waste of energy.
Yes, goldfish in a home really don’t need a heater.
The only warning I’d make about doing a tank for goldfish is this:
A well-cared for goldfish can easily live for 10+ years, and grow up to a foot long. So, while a 10 gallon would probably be adequate for a single goldfish for a long time, you’ll want to plan to get a bigger tank eventually if you want more than one goldie in there.
Contrary to popular belief, “goldfish bowls” are not really suitable for goldfish if you want it to live a normal lifespan.
good luck with your fish. 
NB - all of the above advice is perfect for your standard, run of the mill goldfish.
However, if you’re looking into a fancy fish - a pearl scale or a lion head, they’re quite sensitive (having been interbread to the point of almost non-recognizability as goldfish) - they will require a warmer temp - about 72, at it will have to be constant or they will get stressed and die.
For your standard goldfish, you can actually freeze them for short periods of time and they will come back to life. No, I’m not making this up. Yes, I did witness it with my own eyes.
Just want to second lavenderviolet’s post. A 10g may seem quite large for a single goldfish, but goldfish can get quite large and they’re extremely dirty. A small tank will get polluted fast…which causes problems (I’ll save the elaboration).
Are you just getting a feeder goldfish or a fancy goldfish?
10 Gal. sucks.
Get 20 minimum and maybe bigger if you want soemthing you will enjoy. Plus, with the bigger tank and just a few fish you get to see them really grow.
Also, buy used, save a lot of $$$, and get the filtration system that hangs off the back of the tank instead of the undergravel kind.
(makes life easier)
Goldfish are related to carp, and both live in cool water environments naturally. So as has been said, they don’t need a heater, but they’re more active with warm water. The wild forms of goldfish are actually a dull olive green, and i’ve read that goldfish released into outdoors ponds (wild ones) and streams tend to revert back to their natural coloration.
My parents had a tank with goldfish, i believe it was a 20 gallon. All of the fish died but one, and they had it for years. The thing grew to several inches long and they named it “moby dick”.