Goldfish do best in water temperature around 60 degrees Fahrenheit (15.6 degrees Celsius) but can thrive in a wide range of temperatures from 50 to 68 degrees F (10 to 20 degrees C). It’s best to keep a thermometer on the aquarium to check the temperature. Another way to check the water temperature is to see how quickly the goldfish are breathing.
If the goldfish live in ponds, it is still important to monitor the pond’s water temperature. Use a pool thermometer. Do not use small floating aquarium thermometers because they can be swallowed by koi or other large goldfish, notes Norm Meck of the Koi Club of San Diego. Provide shade from the sun for the fish or they will die from the heat.
When the water is too hot
If an aquarium or pond is too hot, the fish will be at the surface gasping quickly. Warmer temperatures make them breathe faster. The fish are gulping air in order to get enough oxygen to survive. The warmer the water, the less oxygen it contains. Water that is too hot is very uncomfortable for the fish, and they will dart rapidly about, trying to escape. Their metabolism speeds up and they need more food than usual in order to keep from starving. This rapid breathing leads to internal organ damage and kills the fish.
If a tank in a home is constantly between 70 and 75 degrees F, then make sure the tank is not overcrowded because there is not enough oxygen in the water to keep the fish alive, advises “Goldfish: Everything About Aquariums, Varieties, Care, Nutrition, Diseases and More” (Barron’s Educational Series; 2003.) If the aquarium keeps getting too hot, even without a heater, consider purchasing an aquarium chiller.
When the water is too cold
When the temperatures plummet, so does a goldfish’s metabolism and respiration rates. Goldfish should not be fed when the temperature dips below 50 degrees F (10 degrees C) because the food will not be eaten. It will then rot and may cause a potentially lethal ammonia spike.
Before adding goldfish to ponds, make sure that the pond does not freeze solid during the depths of winter. Otherwise, the fish will freeze and die. Some pond owners choose to keep their pond fish indoors in extremely large aquariums during the winter. Goldfish in ponds can grow to be 18 inches long, which makes them far too large for most aquariums.