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How Big Can a Goldfish Get?

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While it’s true that a goldfish will not typically outgrow his tank, this is not because the tank size impedes his growth. Small tanks provide poorer water quality than large ones, and will stunt growth in goldfish, often preventing them from reaching their average size of between 8 and 12 inches. But give a goldfish good quality water and lots of space and you’ll give him every chance of growing to a good size.

Indeterminate Growers

Unlike dogs, cats and other common pets, goldfish grow until they die. They do not grow at a steady, predictable rate; instead, goldfish grow in bursts. Typically, an outdoor goldfish grows more when the weather is warm and less in winter.

Stunting of Goldfish

If goldfish are kept in a poor quality environment, they will not grow. Furthermore, they may develop physical deformities, owing to the severe stunting that occurs. However, putting a goldfish in a large enclosure with great water quality doesn’t guarantee that the fish will grow huge, these conditions just give him a better chance of getting big. Although goldfish grow continuously, genetics influence the potential size of the fish as much as environment.

Giant Goldfish

The longest goldfish to date measured 18.7 inches. The fish, which belonged to a Dutch lady, was never weighed, so his mass remains unclear. However, a goldfish caught in Michigan measured 15 inches and weighed 3 lbs, so it’s safe to assume that Dutch behemoth weighed in excess of 3 lbs. Fancy goldfish typically grow to 8 inches, while single tail goldfish grow to 10 and sometimes up to 12 inches in length. It's rare for domestic goldfish to achieve the monstrous sizes recorded in the Netherlands and Michigan; these fish were exceptions to the norm.

Growing a Big Goldie

Goldfish benefit from warm water, between 71 to 75 degrees Fahrenheit. The fish require a sufficiently large environment. Fishkeeping.com advises a minimum 48-gallon-tank for the first goldfish and 14 more gallons for each additional fish to give them plenty of room to swim and grow.