No, goldfish don’t need a heater. Goldfish are not tropical fish and thus they don’t need warm water temperatures to thrive.
Goldfish can tolerate a wide temperature range, from 50°F to 81°F (10°C and 27°C). In fact, they prefer cooler water, from 68°F to 74°F (20°C and 23°C).
With that said, in some cases, you may need to consider installing a heater in your goldfish tank. First, fancy goldfish are more sensitive to temperature changes compared to common goldfish, so you may need to get a heater if you’re keeping fancy goldfish.
Second, you should get a heater if you wish to breed your goldfish. To breed your goldfish properly, you need to mimic seasonal temperature changes that occur in their natural habitats. This means the water needs to stay between 50°F and 59°F (10°C and 15°C) in the winter and between 68°F and 74°F (20°C and 23°C) in the summer.