The short answer
Goldfish are genuinely cold-water fish and tolerate low temperatures far better than tropicals. A healthy single-tailed goldfish can handle water from near freezing up into the low 20s°C, going dormant as it cools toward the bottom of that range. The real danger isn’t cold itself — it’s a sudden swing. A fast temperature drop shocks the fish, whereas a slow seasonal cooling it can adapt to. Fancies are the exception: they’re noticeably less cold-hardy.
The comfortable range
For everyday keeping, goldfish do best around 18–22°C. They’ll happily live cooler, and in a pond they slow down and stop feeding as the water falls below about 10°C. Toward freezing they become dormant, resting near the bottom where water is warmest and barely feeding. Provided the change is gradual and the fish is healthy, this is normal overwintering behaviour for hardy single-tails, and they’ll perk up again as the water warms in spring.
Single-tails vs fancies
- Single-tails (common, comet, shubunkin): tough and cold-hardy, well suited to unheated tanks and ponds.
- Fancies (fantail, oranda, ryukin): their compact bodies make them more sensitive to cold, and many keepers bring them indoors or add gentle warmth in winter — see do fancy goldfish need a heater?
Avoiding cold shock
The mistakes that harm goldfish are rapid drops — a big water change with cold, unmatched water, or a heater failure. Always temperature-match replacement water, avoid dumping in cold water, and let seasonal changes happen slowly. If you keep goldfish in an unheated setup, read how to set up a cold-water aquarium. For pond keeping, see can goldfish live in a pond?