Cool water is the goldfish climate
Goldfish descend from carp adapted to temperate ponds, and everything about their keeping follows from that. In water around 18 to 22 degrees they feed, grow and resist disease well; push the tank into the mid-twenties with a heater and you speed their metabolism while warmer water holds less dissolved oxygen — a double squeeze on a fish already famous for producing heavy waste. Fancy varieties with deep bodies are the most vulnerable to that squeeze, developing buoyancy and digestive trouble in water a tropical fish would call perfect. So the heater question for goldfish answers itself in reverse compared with their tropical tankmates-in-waiting at the shop: do not add one, and never house goldfish in a heated tropical community where the temperature that suits the other fish quietly works against them every single day.
What goldfish need instead of heat
Spend the effort a heater would have taken on volume, filtration and stability. A spacious tank with strong filtration sized above the tank's rating handles the waste load that actually limits goldfish health, and a thermometer still earns its place — not to trigger heating, but to warn you when a hot room or a sunny window is pushing the water warmer than the fish want. Keep the tank away from radiators, conservatories and direct sun, and in a summer heatwave use room shading, extra aeration and lights-off hours to hold the line, since warm, still water is when oxygen crashes happen. Replacement water for changes should match the tank's cool temperature; the classic goldfish chill accident is a winter change done with warm water from the tap followed by a slow drift back down.
The narrow exceptions, handled carefully
There are two situations keepers mistake for a general rule. Outdoors, pond goldfish overwinter fine under ice in a deep enough pond and need depth and a hole in the ice for gas exchange, not heating. Indoors, a fancy goldfish kept in an unheated room that drops towards freezing in winter may justify a heater set very low purely as a frost guard to hold the high teens — that is preventing a plunge, not warming the fish, and it is the only indoor use that makes sense. Breeding conditioning sometimes raises temperature gradually by a degree or two in spring to mimic the season, a deliberate short programme for experienced keepers, not a reason to run a heater year-round on a pet tank where steady cool water is the whole point.
Temperature trouble in a cool-water fish
A goldfish in water that has crept too warm typically hangs near the surface or the filter outflow where oxygen is richest, breathes fast, and loses appetite despite the faster metabolism. Cool the room side of the problem gradually, increase aeration, and test the water, because warm water and waste spikes travel together in goldfish tanks. A fish that goes off balance, floats oddly or stops feeding for more than a couple of days deserves an aquatic vet's assessment with the temperature history included. Recording that history — the tank's normal cool range and any heatwave readings — in KinsLe in the Pet Passport gives the vet the context that separates a temperature story from a disease story.