Goldfish: cold water, heaters and the ideal temperature
Published by Planète Pets · managing editor: Clément Lacaille · how our guides are written
People keep saying the goldfish is a "cold-water" fish, and that is true: unlike tropicals, it does not need water held at 25 °C. But cold water does not mean freezing water, nor a temperature left to the mercy of the seasons. Temperature drives its metabolism, appetite and immunity, as shown by work on the effect of temperature on goldfish physiology. Find our other pointers in the bedding and habitat section.
Contents · 5 sections
What temperature to aim for
A range of 18 to 22 °C suits most pet goldfish. Below 10 °C the fish slows right down and barely eats; above 24 °C the water loses oxygen and the metabolism races. What matters is stability: large daily swings tire the fish far more than a slightly cool but constant temperature.
Does it need a heater?
In a heated home, a goldfish aquarium often does without a heater, because the room already holds 18 to 21 °C. A heater set low (around 18-20 °C) becomes useful in a cold room or to avoid winter swings. If you install one, pick it sized to the volume and reliable: our guide aquarium heater and thermostat helps you get it right, and a simple thermometer stays essential for monitoring.
Avoiding thermal shocks
A goldfish tolerates a range of temperatures, but not abrupt swings. During a water change, the new water should be close to the tank's temperature. A gap of several degrees at once causes stress, sometimes white spot in the following days. Gentle transitions matter as much as the figure on the display.
Hot summers
Summer heat is the real danger: above 26-28 °C the water holds less oxygen and the goldfish gasps at the surface. Our guides cooling the water in a heatwave without shock and low oxygen in warm water cover the moves that save fish: surface agitation, ventilation, never an ice cube dropped in all at once.
Frequently asked questions
Does a goldfish need a heater?
Not always: in a heated room, the ambient temperature is enough. A heater set low helps in a cold room or to smooth out winter swings.
What is the right temperature for a goldfish?
Aim for 18 to 22 °C, stable. Extreme cold makes it lethargic; heat above 24 °C strips oxygen from the water and stresses it.
Can you keep a goldfish with heated tropical fish?
No: their temperature needs differ too much. The cold-water goldfish has no place in a tropical tank held at 25-26 °C.
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This guide is part of Planète Pets’s Fishkeeping universe. Our advice is general in nature: for any health concern, your veterinarian remains the only reference.
