5 Fascinating Facts About Goldfish

Goldfish are one of the most popular beginner pet fish and require special care and considerations. Here are five amazing facts about goldfish.

1. Goldfish Live 15–20 Years

Ever wonder, “How long do goldfish live?” Pet goldfish are among the longest-living species of fish. Depending on the variety and standard of care, some can easily live into their 20s.

A goldfish’s lifespan depends on their genetics, diet, and environment. Like all other pets, optimal water quality and well-rounded diets go a long way to ensure any pet can reach their full potential and benefit from the greatest overall health throughout their lifetime.

2. Most Pet Goldfish Reach 12–16 Inches in Length

Another commonly asked question about goldfish is “how big do goldfish get?” Goldfish can grow up to a foot in length or more. Long-bodied goldfish can easily grow to the size of a dinner plate and beyond, while fancy varieties will typically not grow as large but may have the potential to grow larger if given enough room and protein in their diet.

The most important factor for goldfish to grow properly is their tank size. The bare minimum tank size for a goldfish is 20 gallons per fish, and the tank can easily increase to 50 gallons per fish by the time they reach full length.

A fish “growing to the size of their container” is the same as stunting them. The constant stress of a tiny environment causes decreases in their growth, reproductive capability, and immune function. A small tank also causes significant health issues for a fish, including a shortened lifespan.

3. Goldfish Like Friends—But Not Always

When keeping goldfish, it’s important to consider other good tank mates. Goldfish not only do well when kept in a school, where they swim behind one another in a group, but they also do well in pairs or as single fish.

If you have a fish that has been by themselves for a while, however, they may not appreciate having new fish added—especially if their new tankmates are smaller. “Only child” goldfish have been known to be aggressive to new tank mates who disrupt their happy home. Single goldfish also need more attention and stimulation from their human companions, so try to spend time with them every day, offering food or tank enrichment.

It's not recommended to mix standard and fancy varieties of goldfish. Fancy varieties don’t swim very well, and there can be too much competition during feeding time, which often results in the fancy goldfish not getting enough to eat.

If you don’t have room to keep multiple goldfish tank mates—or have a single fish that cannot tolerate other goldfish—you may want to consider other temperate fish or mollusks that don’t require a heater to keep your goldfish company. These species include zebrafish, ornamental minnows, and snails.

4. Goldfish Can Eat Many Different Foods

So, what do goldfish eat? All goldfish should be on a pelleted diet, because pellets have more nutritional value than compressed flakes. Replace your fish’s container of food every six months to ensure adequate vitamins, such as water-soluble vitamin C.

You’re also welcome to feed your goldfish an occasional (once or twice a week) treat of high-protein shrimp or tofu, fruits, or vegetables.

What goldfish eat can significantly impact their overall health and longevity, as well as how fast they grow. Because they are carp species, goldfish have amazing growth potential, but you don’t want to max out your tank space quickly.

It’s important to pay close attention to how much protein your fish is eating. High protein in the diet will result in a fast-growing goldfish with lots of waste in your water. Juvenile fish under 4 years old should receive a higher-protein diet, around 35%. This level also applies to any goldfish that are breeding. A protein level around 30% is a good level for maintaining your goldfish’s current size.

Unless they’re juveniles and still growing or breeding more fish, goldfish do not need much fat in their diet. High-fat diets (above 8%) have led to fatty liver disease that can cause death.

5. Goldfish Sleep in ‘Low Power Mode’

You may have seen goldfish hover just off the bottom of their tank, with their gills and mouth still moving. But are they sleeping? Do goldfish sleep?

Goldfish do sleep—but it doesn’t look the same as human sleep. Because they’re prey animals in the wild, most fish cannot be fully asleep without monitoring their environment.

When goldfish sleep, their gills, mouth, and eyes continue to move in order to breathe. They may look like they’re awake, but they’re in “low power mode,” meaning many of their functions are shut down, but their brain is still monitoring the environment for potential attacks.

Some fish, including goldfish, may have a favorite spot in their tank that they like to sleep. This may be a corner of their tank that has less flow to swim against, inside their favorite plant to provide an anchor, or in a piece of decor.

Fish require light and dark periods, just like humans do. This can be provided by ambient lighting from a nearby window or a light on top of their tank. Try to match the normal outdoor light and dark cycles with a timer, or even better, a dimmer. Some fish can have a startled reaction to a sudden increase in light, so a gradual “dawn” and “dusk” light cycle is better for all fish, including goldfish.