Keeping and selectively breeding goldfish for ornamental purposes goes back over 2,000 years to China’s Tsin Dynasty (265–419 A.D.)1. After much debate, scientists have concluded that the pet fish we know today is a domesticated carp (Carassius auratus) from eastern Asia. In the wild, it can reach lengths of nearly 20 inches. Of course, your pet is unlikely to get that size. The average is 8 inches.2
However, many factors will determine how big your goldfish gets. Some involve genetics, while others are environment and health-related. Let’s do a deep dive into the elements controlling growth and development.
Originally, the goldfish was drab colored, which was essential for a prey species. It provided camouflage to help it evade predators in the ponds, lakes, and rivers of its native habitat. The species is an opportunistic omnivore that prefers cool waters. That’s also true with captive fish. It forms schools that satisfy its social needs and provides a defense against predators.
The goldfish is relatively long-lived, both in the wild and in captivity. Interestingly, it typically lives longer in its native habitat than in an aquarium. Its lifespan is 20 to 30 years versus the 5 to 10-year average in captivity. The difference is telling, which we’ll discuss later in detail.
Goldfish Size and Growth Chart
These data come from captive-raised fish and may not be representative of wild goldfish, which grow faster. A 1-year-old wild specimen may reach up to 7 inches. It can reach a maximum of 18 inches long and weigh 4.5 pounds as an adult.
It’s also worth mentioning that you may not observe the same pattern with your pet, particularly if their variety is selectively bred to be smaller. The other factors rest with your tank setup.
When Does a Goldfish Stop Growing?
Wild goldfish don’t stop growing. A British angler reeled in a 16-inch, 5-pound fish in Dorset, England, in July 2010. We can’t say the same thing about captive goldfish, although growth slows after reaching adulthood. However, the adage that a fish grows with the size of the tank is a myth. The truth is more complicated and involves other considerations.
You may notice that your goldfish has stopped getting bigger. Enthusiasts refer to this condition as stunted growth. Your pet’s wild counterparts hold the key to its occurrence.
Factors Affecting the Size of Goldfish
We mentioned genetics as a factor in the size. For example, Fantail Goldfish probably won’t get more than 8 inches. Therefore, the variety affects its genetically determined length. Your only control over that element is the decision of what kind of fish to buy. Other aspects are malleable. The critical elements include the size of the aquarium, water conditions, and diet.
Using the six-times rule, your average goldfish needs a tank at least 48 inches L x 16 inches W x 16 inches D or about a 75-gallon aquarium. Remember that wild fish get larger and don’t stop growing because they have the room to do so. The other consideration is the resulting water conditions. They are more stable in larger quantities. Vegetated water bodies offer a bounty of food.
Goldfish are messy and voracious eaters. That makes providing the ideal conditions for growth difficult. It can also explain the stunted growth some pet owners may observe. The average hobbyist may not be able to supply what these fish need to reach their genetically determined size. That explains the size and growth rate differences that exist between the two populations.
Many pet goldfish live in the wild, whether intentionally or unintentionally. These fish can reach sizes rivaling wild specimens. Therefore, stunted growth is reversible once it has the necessary space. Bodies of water larger than a tank have more stable water conditions that support this result.
Ideal Diet for Maintaining a Healthy Weight
We mentioned that goldfish are omnivores. In the wild, they consume aquatic vegetation, insects, and invertebrates. Pondweed (Potamogeton sp.) forms a significant portion of their diet. Also, mosquitoes are one of the main insects they take, with crustaceans only comprising a small part of their daily intake.
Research has shown they require a diet of 29% protein and 13% fat with an energy-to-protein ratio of 9.7 kcal to each gram of protein. You should feed your goldfish a commercial diet formulated specifically for this species. You should also provide vegetation such as blanched greens. It’s essential to limit the amount you offer them, being opportunistic feeders.
How to Measure Your Goldfish
The correct measurement for a goldfish includes the snout to the end of its caudal or tail fin. Doing it is straightforward. Surprisingly, goldfish are intelligent creatures that retain memories. You can teach your pet to take food from you, which can help you get more accurate measurements.
Goldfish have brought joy and beauty to humans for hundreds of years. They are adaptable and intelligent fish. The species is more likely to reach its full growth potential in the wild than in your aquarium. Space, water conditions, and a healthy diet are the main factors determining a fish’s size. A tank may offer a decent place to live, but it can’t replicate what it can find in the wild.
Featured Image Credit: Julia Vashurina, Shutterstock