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How Big Do Goldfish Get? Size Guide & How To Help Your Goldfish Grow

Goldfish are sometimes thought of as growing to just a few inches long, but in reality how big do goldfish get? The answer’s larger than you may think!…

By Julie Millis
Last updated on

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how big do goldfish get

How big do goldfish get? Well, the world’s longest goldfish was recorded at 18.7 inches in the Netherlands in 2003 [1]. But how big can you expect your goldfish to grow?

Our team has kept many varieties over the years, and while many goldfish may grow to an average total length of around 6.0 inches in an aquarium, this size is just a guide.

Several factors can influence how big your goldfish gets including its breed, environment, and diet. For example, shubunkin goldfish can get up to 18.0 inches in a pond!

So read on if you want to find out how big different goldfish varieties can get, and how you can give your goldfish the best chance of reaching its full growth potential!

How Big Do Goldfish Get? Maximum Sizes Of Popular Breeds

With around 300 breeds recognized globally, you’ll find there can be a great deal of variation in how big goldfish get.

Whilst there are other common varieties such as Veiltails and Bubble Eye goldfish, the table below summarizes the maximum sizes of some of the most popular types.

Goldfish BreedMaximum Size (Inches)
Common8.6 to 16.1* (SL) [2,3,4]
Comet6.0 to 12.0 (TL) [5]
Shubunkin5.0 to 18.0* (TL) [6]
Fantail6.0 to 8.81 (SL) [7]
Oranda7.0 to >12.0* (TL) [8, 9]
Ranchu5.0 to 8.0, >12.0* (TL) [10]
* denotes outlier sizes typically found in larger environments such as ponds.

You’ll notice these measurements are given in Standard Length (SL) or Total Length (TL). Standard Length is often used with bony fish as it measures the tip of the snout to the end of the body or last vertebra. Total Length measures from the tip of the snout to the end of the tail fin.

It’s worth bearing SL vs TL in mind when looking at how big goldfish get. Many sources don’t distinguish between the two, hence you can find different lengths for the same fish. 

Now let’s look at how big popular hardy breeds vs fancy goldfish can get in a little more detail.

How Big Do Common Goldfish Get Vs Comet Goldfish?

Also known as feeder goldfish, the Common goldfish is one of the best-known hardy varieties and can grow to a body length (SL) of between 4.7 to 8.6 inches in an aquarium [11], or over 10 inches including the tail. They can grow up to 16.1 inches (TL) in a pond [12, 13].

The Comet goldfish originated in America in the 1800s and is still one of the most popular breeds in the US today. While it has similarities with the Common goldfish, it has a more slender body and is characterized by its distinctive flowing tail.

Comet goldfish generally grow to a maximum size of around 6.0 to 10.0 inches (TL) in an aquarium including their tail. Although, in a larger environment they may grow to 12.0 inches (TL) [14].

How Big Do Fancy Goldfish Get?

With many more varieties amongst fancy goldfish, there can be even more variation in how big these goldfish get. Generally though, they are shorter and rounder than single-tailed hardy goldfish.

Many fancy goldfish can grow to a maximum size of between 6.0 to 8.0 inches (TL) in a fish tank, but there are outliers. Ranchu and Oranda can grow to over 12.0 inches (TL) in an outdoor pond. Although Ranchu’s are very sensitive to temperature so often do better indoors.

How Big Do Goldfish Get In Different Environments?

So we’ve talked about the maximum sizes popular goldfish breeds can achieve, but what are the more realistic average sizes, and how big do goldfish get in different environments?

comet goldfish in aquarium

How Big Do Goldfish Get In A Fish Tank?

There are many factors that affect how big your goldfish will get in an aquarium. But as a guide, most goldfish are likely to grow to an average size of around 4.0 to 6.0 inches (TL) in a tank.

However, one factor that’s often the subject of much debate when it comes to how big goldfish get in a fish tank is to do with growth-inhibiting hormones (GIH).

Growth Inhibiting Hormones In Goldfish

Your goldfish produces two key substances, amongst several others, that help regulate its growth. These are gamma-aminobutyric acid (GABA) and somatostatin (aka growth hormone inhibiting hormone or GHIH). 

GABA is an amino acid that inhibits growth hormone (GH) in goldfish [15], and somatostatin is a hormone that also inhibits GH [16,17,18]. Growth hormone itself is an important substance your goldfish uses to stimulate its metabolism and growth [19].

Many believe that goldfish secrete GABA and somatostatin into their water and a build-up of these substances helps stop them from outgrowing their tank.

Hormones are substances that work inside the body, whereas those secreted into the environment are pheromones. Despite this, studies have shown that goldfish can secrete some hormones into their environment [20], but not specifically somatostatin.

We’re not looking to debunk the theory that GABA or somatostatin builds up in your tank’s water to inhibit goldfish growth. But we are highlighting that scientific evidence for the release of these specific substances into their water by goldfish is lacking.

How Big Do Goldfish Get In A Pond?

Generally, hardy goldfish breeds including Common goldfish and Comets will grow to an average size of about 10.0 to 12.0 inches (TL) in an outdoor pond. This can also be true for some fancy breeds, although in general, it’s less likely they’ll reach this size.

Keeping your pond water well-filtered and aerated, as well as providing high-protein food as part of their diet can help your goldfish grow and thrive outdoors.

How Big Do Goldfish Get In The Wild?

Goldfish that have been released into the wild can reach a size of anywhere between 12.0 to 16.0 inches (TL), but some have been found up to 18 inches (TL) [21].

An open water habitat, like a large lake, not only provides them with an abundance of space, but regular rainfall brings a supply of naturally oxygenated freshwater. This coupled with protein-rich natural food sources, such as fish eggs and larvae, can allow goldfish to grow very large indeed.

Should You Release Goldfish Into The Wild?

In short, no! Goldfish are a nonindigenous invasive species that have had many negative impacts on lakes and other habitats across North America. These include outcompeting and reducing natural fish populations, and impacting and reducing vegetation [22, 23, 24].

If you can no longer care for your goldfish, consider contacting your local pet store, another aquarium or pond owner, or look for a local aquarium society to try to rehome your pet.

shubunkin goldfish

Goldfish Growth Rate

Goldfish fry grow rapidly, reaching around 1 inch in the first month to about 2 inches at 6 months. After this, a goldfish’s growth rate slows to around 1 inch per year.

How Long Does It Take Goldfish To Reach Full Size?

While goldfish are considered adults at around 7.5 months when they become sexually mature [25], it’s very difficult to predict when they’ll reach full size. 

Goldfish continue to grow throughout their lives and environmental factors, diet, and their breed all have a bearing on when they’ll reach full size. For example, goldfish kept in ponds outdoors will tend to grow more in the warm weather of spring and summer, rather than in winter [26].

Average Weight Of A Goldfish

Again, the average weight of your goldfish will depend on several factors including variety. But a goldfish of around 6.0 to 8.0 inches (TL) can weigh from about 3.5 up to 10.0 ounces [27]. If you have a larger goldfish in an outdoor pond, it is obviously likely to weigh more.

Why Do Goldfish Get So Big? The Importance Of Genetics

Although we know goldfish were first bred in China over 1000 years ago, their exact genetic origin has been the subject of debate. 

They have previously been thought to be descended from the Prussian carp (Carassius gibelio), although modern studies have shown them to originate from the Crucian carp (Carassius carassius) and Chinese goldfish (Carassius auratus auratus) [28, 29, 30].

Crucian carp have a similar average length to goldfish of around 6.0 inches, although some larger specimens have been reported in males. This may explain why goldfish have a similar average size.

Genetics is also important when it comes to choosing your goldfish. Feeder goldfish that have been subjected to inbreeding are unlikely to grow to a large size. So it’s important you look to buy your goldfish from a reputable fish store or breeder if you want to ensure optimal growth.

What Causes Lack Of Growth In Goldfish?

Several factors can cause a poor rate of growth in your goldfish, as well as affect its overall health. Let’s look at the most important ones in more detail.

lionhead goldfish

Poor Water Quality And Temperature

Keeping your goldfish in poor water conditions can not only inhibit its growth but can also be fatal. As with most fish, the build-up of ammonia and nitrates in a dirty tank can be toxic in a relatively short space of time [31].

A poorly-aerated fish tank can also hinder your goldfish’s growth as they need plenty of oxygen for their metabolism. Studies have shown that goldfish will ‘gulp’ air from the surface when there are depleted oxygen levels in the water [32].

Water temperature can also affect your goldfish’s metabolism and growth. If the temperature falls to around 59°F or below, it can cause a significant drop in metabolic rate [33]. In fact, hardy goldfish kept in an outdoor pond will often begin to hibernate around these temperatures.

Not Enough Space To Grow

A tank that’s too small or overcrowded can also affect your goldfish’s growth rate and general health [34]. Goldfish generate a lot of waste, and excess feces in an overcrowded tank can affect your water quality.

Overcrowding can also lead your goldfish to become stressed, which can not only affect their growth but can also cause diseases such as hemorrhagic septicemia [35]. This disease can cause internal bleeding in your goldfish and is often fatal.

Poor Diet And Overfeeding

Goldfish are omnivores and need a balanced diet containing the right levels of nutrients including protein and fats. In fact, a diet of less than 4.5% fat can reduce growth and lower its survival rate [36].

Overfeeding can be a major problem in goldfish too. While feeding them the right amount at the right time can help optimize their growth [37], overfeeding can have the opposite effect. Leftover food can also collect at the bottom of your tank, again leading to excess ammonia.

Disease In Goldfish

Like other freshwater fish, goldfish are susceptible to several diseases that can hinder their growth and overall health. Parasites like anchor worms or flukes, such as Gyrodactylus kobayashii, can sometimes become an issue in an overstocked tank [38].

Fish lice (Argulus spp.) are another common parasite that can affect growth in goldfish [39]. One of the best ways to prevent these and other diseases is to quarantine new fish for 1 or 2 weeks before adding them to your tank.

Caring For Your Goldfish So They Grow To Full Size

So how can you help your goldfish reach its potential full-size? Take a look at some of the key factors you can influence if you want your fish to grow.

Tank Size And Stocking Level

One of the best ways you can influence your goldfish’s growth is to keep them in a large enough aquarium that’s not overstocked. In general, you should start with a 20 gallon aquarium as a minimum for a single goldfish, and add 10 gallons for each additional fish.

By stocking your tank correctly your goldfish are less likely to become susceptible to diseases associated with overcrowding, and a larger aquarium also helps you have better control over water parameters. Both of which can help improve the growth of your goldfish.

Water Quality And Temperature

Keeping your water parameters in check can have a significant impact on your goldfish’s growth so monitor your levels and perform water changes at least weekly. Ideally, pH should be around 6.5 to 7.5, with ammonia less than 0.1 ppm.

Goldfish prefer a tank with a lower flow, so an adjustable filter with the right biological filter media is one of the best ways to reduce ammonia and other harmful toxins in your tank. You should also provide good aeration as high-oxygen levels will support your goldfish’s metabolism.

The best temperature for fancy goldfish is around 75°F, with hardy breeds preferring a degree or two lower. Too high and your goldfish can become sluggish, but too low and its metabolic rate will fall, inhibiting growth.

fancy goldfish feeding

Diet And Nutrition

While goldfish need a balanced diet that is usually high in carbohydrates, you can increase growth by giving them more protein and fat. A diet of around 40 to 45% protein can significantly increase growth [40], especially in your goldfish’s first few months.

You can increase your goldfish’s protein levels by feeding them food like bloodworms, daphnia, and brine shrimp. But make sure you still provide a high-quality pellet or flake food to keep their diet balanced.

The amount your goldfish eats, and even the time of day when it feeds can affect its growth [41]. To avoid overfeeding, feed him twice a day, ideally first thing in the morning and last thing at night, and only provide what he can eat within 2 to 3 minutes.

What To Do Next?

So how big do goldfish get? Depending on their breed, environment, and several other factors you can see they can get pretty large indeed!

While most goldfish kept in an aquarium may reach around 4.0 to 6.0 inches (TL) there are steps you can take to give yours the best chance of growing to a larger size. 

Make sure you choose your goldfish from a reputable fish store or breeder, feed them a balanced, high-protein diet, and provide them with a large enough aquarium.

If you want to know more, check out our full guide to the best goldfish care for tips on how to look after your fishy friend!

Frequently Asked Questions

So how big do goldfish get? Take a look at the answers to some of the most frequently asked questions!

How Big Do Goldfish Get In An Aquarium?

While there are many factors that can determine how big your goldfish will get in an aquarium, most goldfish will grow to around 4.0 to 6.0 inches (TL).

How Can I Make My Goldfish Grow Faster?

Keep a single goldfish in a 20 gallon tank at a minimum, provide well-filtered, clean water, and include about 40 to 45% protein in his diet with foods like bloodworms. 

How Long Do Goldfish Live?

A typical lifespan for several goldfish breeds is between 5 to 10 years, although in some cases it can be up to around 20 years.

How Big Was The Biggest Goldfish Ever Recorded?

The longest goldfish was recorded in the Netherlands in 2003 at 18.7 inches! [42]

Photo of author
Julie Millis
Julie has been involved in aquatics for over 15 years. She is passionate about freshwater and saltwater tanks. Julie loves helping with all your fish-keeping questions!

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