Goldfish, like many living creatures, react differently in different temperatures. However, goldfish physiology changes so drastically as the temperature drops that they can not only live weeks or months without food, but food may be dangerous to them. Their metabolism slows, and all activity works to match the outside environment.
Goldfish in warm water
In what is considered warm water to a goldfish—that is, 65 degrees Fahrenheit or above—a goldfish is active and inquisitive. It is on a constant scavenger hunt for bits of debris, algae, and creatures that are small enough to fit in its mouth. The fish is territorial, and will frequently challenge any other fish of comparable or lesser size within what it deems to be its space. If it’s in an aquarium or landscaped pond, the fish may further delineate its own territory by moving rocks and plants.
At this warmer temperature, the goldfish is an opportunistic eater that will swallow anything that’s even remotely edible. It defecates anywhere from once a day to several times a day, depending on how much it’s eating. All this activity means that it will absorb more of the dissolved oxygen in the water. The fish can grow at incredible rates in these warmer waters.
Behavior and physiology in cold water
Cold water tends to hold less food than warm water. More importantly, it also holds less dissolved oxygen than warmer water. These two factors dictate much of the fish’s behavior and physiology. In this context, “cold water” generally refers to anything under about 50 degrees Fahrenheit, though these changes will be apparent to a lesser extent whenever the temperature dips below 65.
A cold goldfish goes into something similar to a hibernation mode. All of its activity slows, and it will often simply float a little above the ground, moving only enough to keep itself in place. Goldfish have been known to actually freeze solid in ice, and still resume normal functions after they’ve thawed. Often, the deciding factor in the fish’s survival in such extremes is whether or not it can move its gills.
As all of the fish’s internal processes slow, so too does its need for food. In fact, a goldfish that’s fed in very cold temperatures may become ill or die as the food rots in its gut before it can be digested. This is why pond fish do just fine without feeding throughout the winter, and are ready to eat ravenously when the weather warms.
In colder water, the goldfish will not grow, or will grow extremely slowly. This allows it to use the least possible amount of food and oxygen through the winter, and then experience a massive growth spurt in the spring right before spawning.