How Big Do Goldfish Get?

how big do goldfish get

How Big Do Goldfish Get?7 mins read

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Tal Halperin
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How big do goldfish get?
Image from Flickr

So, just how big do goldfish get? The average size of a goldfish depends on many factors and in fact, there is still a lot about these fish that is unknown to science in terms of how they fare in the wild. These fish were originally ornamental fish that developed from captive carp in China. Therefore, it is unpredictable as to how they will thrive in different conditions and this includes their size and growth rate. Ultimately, ‘how big can goldfish get’ is a very open question.

In truth, we know that the maximum size for goldfish in captivity is roughly 12 inches or 30 centimeters. However, feral populations that have taken over lakes and rivers across Europe show different growth patterns to those in captivity. This article will show how goldfish growth in general can vary drastically based on the environment. Moreover, we’ll look at ways to optimize your goldfish size, as well as how big do comet goldfish get compared to their fancy cousins…

What Factors Affect Goldfish Size?

What factors affect goldfish size?
A happy, healthy, and well-kept comet goldfish can grow up to 12 inches or 30 centimeters.

1. Breed

Whilst the word ‘goldfish’ often brings to mind a common or comet goldfish of the classic ‘fairground’ variety, there are actually a huge number of varieties of these fascinating fish, in all sorts of colors. If you’re wondering ‘how big do goldfish get?’ it helps to know what kind of goldfish you have in the first place – and most importantly, whether it is fancy or hardy.

Hardy goldfish include the common goldfish, although the comet goldfish is slightly more popular, and the rest tend to fall under the umbrella category of ‘fancy goldfish’, although there is lots of variation within this.

How Big Can Comet Goldfish Get?

The comet goldfish is one of the most popular hardy goldfish, partly for their distinctive forked tails and partly for their delightful variety of colors ranging from black, calico and orange to the red and white sarasa comet. If you have wondered ‘how big do comet goldfish get,’ you will actually find these are one of the larger goldfish varieties.

So, how big do comet goldfish get? Comet goldfish can grow to very large sizes in a tank. Furthermore, those you keep in a pond can grow even larger. This is due to the increased space and enriched environment.

A happy, healthy, and well-kept comet goldfish can grow up to 12 inches or 30 centimeters and this is marginally more likely if you keep your fish in a pond as opposed to a tank. One of the largest comet goldfish in the world grew to 13-14 inches and weighed 2 pounds!

How Big Can Fancy Goldfish Get?

What distinguishes fancy goldfish from hardy goldfish? Whilst even among fancy goldfish there are varying hardiness levels, one of the major distinguishing factors is an egg-shaped body.

This genetic mutation first appeared in the Song Dynasty in China and has a number of significant effects on goldfish. For starters, fish with an egg-shaped body have less space for their swim bladder. This makes it more sensitive and means they expend more energy on staying afloat.

Unfortunately, this can stunt their growth size compared to hardy goldfish. This is also why fancy goldfish are generally smaller than your average comet or common goldfish. Therefore, ‘how big can goldfish get’? depends on the exact breed.

Furthermore, the mutation itself leads to the body length of your fish being shortened. This is another reason why fancy goldfish don’t grow as big.

Fancy goldfish can grow up to 8-9 inches (20 centimeters) in their hardiest varieties, such as the fantail goldfish. More sensitive fancy goldfish like the celestial or bubble-eye goldfish grow up to 5-6 inches or 15 centimeters.

2. Tank Size

Tank Size of goldfish
You should have a tank that is at minimum 30 gallons or 114 liters for fancy goldfish.

Tank size is one of the biggest deciding factors in how big your goldfish gets. It is a myth that goldfish grow to match the size of their tank. Instead, you should have a tank that is at minimum 30 gallons or 114 liters for fancy goldfish and even larger, at 50 gallons or 179 liters for hardy goldfish like the comet.

Tank shape can help reduce goldfish stress as well. A rectangular tank that is longer than it is tall and gives your fish plenty of room to swim up and down is perfect.

Goldfish keeping tips:

Many fishkeepers recommend adding an extra 10 gallons of water to the tank for each additional fish. This is true, but as tank density also affects how big your fish grow and their stress levels it doesn’t hurt to go bigger and add an extra 20 per fish depending on how many you are planning on getting.

Goldfish prefer to be kept in groups, generally of up to 5-6 depending on the variety. Therefore if you are asking ‘how big can goldfish get’ or ‘average goldfish size’ before you get your fish and set up your tank, you are on the right track. A well-prepared aquarium is always better for your fish, will make things easier for you, reduce their stress, and ensure they reach their maximum size.

Remember that with changes in tank size also come changes in filtration: the ideal parameters are as follows:

  • A flow rate of 10x tank capacity for hardy goldfish
  • A flow rate of 4 or 5x tank capacity for fancy goldfish

If you’re wondering specifically how to look after comet goldfish vs fancy goldfish like the black moor, remember that these fish have subtly different requirements in terms of tank set up, and this can also affect the sort of tank you get.

3. Stress

Stress in goldfish
Poor water quality can lead to signs of stress and sickness such as your goldfish not eating.

You may already know that fish kept in a small tank tend to get stressed. However, many factors can affect the stress levels and thus the growth of your fish. One of these is not just tank size but also tank density.

In fact, keeping a low density of fish in your tank actually improves their growth rate. Not only is there less competition between fish for food, but fish will have a cleaner environment too. Lower fish density also has a positive impact on water quality. This is one of the biggest factors in fish immune stress. Indeed, poor water quality can lead to signs of stress and sickness such as your goldfish not eating.

4. Diet

Diet for goldfish
Feeding your fish a balanced diet can help them reach their maximum size and stay free from disease. Image from Flickr

Feeding your fish a balanced diet can help them reach their maximum size and stay free from disease. This ideal diet mimics the conditions of slow-moving streams and ponds. These environments are ones that carp would have experienced in the wild.

You may have become used to the idea that pet store goldfish can simply be fed with pellet food. Whilst these foods are great for ensuring your fish’s diet is nutritionally balanced, there is more to it. To optimize it, these guys need protein and vegetables too.

Depending on how hardy your fish is, you can feed either live, free swimming protein like daphnia or tubifex worms, or frozen protein. In addition, vegetables like half a shelled pea, little bits or lettuce leaves, or small broccoli florist are perfect to keep your fish’s diet varied.

However, it’s not just what you feed your fish, but also how frequently you feed them that can influence their growth rate. Science has found that fish fed little and often grew more rapidly than those fed the same amount of food in just one sitting. Likewise, a high protein diet can increase the growth of the wen in hooded goldfish types like the ranchu.

5. Sickness

Sickness in goldfish
It’s important to be able to spot signs of infection, like your fish changing color, swimming unusually, or even showing signs that it is close to death.

Any kind of sickness will inevitably stunt the growth of your fish. This is because a fish only has a certain amount of energy to fuel itself. Therefore, a fish that is expending energy on fighting off infection won’t have the ability to put that energy into growth. It’s important to be able to spot signs of infection, like your fish changing color, swimming unusually, or even showing signs that it is close to death.

However, sometimes sickness can be a vicious cycle. This is especially if your fish catches them while still young. Fish that are undernourished or have their growth stunted will likewise in turn be more susceptible to infections further down the line. And, as they are smaller, they are more likely to be outcompeted by tankmates that didn’t get sick.

If this has happened to any of your fish, rest assured that there are ways to deal with it.

Feeding an enriched diet with a balance of vegetables and proteins as opposed to just pellets can help. However, there are also ways you can supplement your fish to boost their immune strength and growth. For example, spirulina algae is not only great for your fish’s color. On top of this, it has also been proven to help many species defend against diseases too.

6. Genetics

Genetics of goldfish
Just like how people vary in height, even within the same variety of goldfish, some fish simply grow larger than others due to genetics.

Just like how people vary in height, even within the same variety of goldfish, some fish simply grow larger than others due to genetics.

There’s not much you can do to predict the full size of your goldfish. In the end ‘how big do goldfish get’ might need the answer ‘depends on the individual goldfish!’ Nevertheless, by optimizing their tank size, stress levels, and diet, and by reducing sickness, you can maximize its genetic potential.

Conclusion

how big can goldfish get
It is a common myth that goldfish grow only as big as their tank allows. Image from Flickr

There are many myths and misconceptions about the size goldfish can grow to. When these pets first became popular, not as much was known about their well-being. As a result, practices developed that are now considered cruel, like keeping them in bowls. With more knowledge about these mesmerizing creatures, it’s now possible to help your fish reach the maximum size. Not only this, you will help it live a healthy and happy life in which it thrives.

Frequently Asked Questions

How big can goldfish get in a tank?
How big goldfish grow in a tank very much depends on the size of the tank in question. The most commonly recommended tank size for goldfish is a 30-gallon or 114-liter aquarium, but for some varieties such as the comet or common goldfish, this is insufficient and a 50-gallon or 189-liter aquarium is recommended instead. If given this amount of space goldfish can get to 12 inches or 30 centimeters. However, tankmates are also a factor and if a tank is overcrowded your goldfish won’t grow as there simply aren’t enough resources for them.
Do goldfish grow to match the size of their tank?
It is a common myth that goldfish grow only as big as their tank allows. The truth is, this may look like the case, but in reality, your fish’s growth is stunted and they will be stressed and more prone to diseases. This myth started in the early days of fishkeeping when people didn’t know as much about these popular pets and it was common practice to keep them in a small tank or even a bowl. However, this myth is still sometimes found and it’s important to realize that goldfish are complex and dynamic creatures that need space to grow to their full capacity.
How fast do goldfish grow in a pond?
There isn’t much of a difference between how fast goldfish grow in a pond vs in a tank, although it can be argued that for some kinds, especially hardy goldfish like the comet, a pond is an enriched environment that lowers stress and boosts their immune systems, meaning they might grow marginally faster as they are healthier. What a pond does influence is your fish’s ability to reach its full size - up to 12 inches or 30 centimeters - as well as the ability to live a long life. Many fish pond owners find that their hardy goldfish thrive when in a pond and live much longer than expected.
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Sydney Perry

Sydney Perry has loved fish since she was a child and has enjoyed keeping many varieties over the years, ranging from black moors and shubunkins to betta fish. As a lover of nature and of Japanese culture, her dream tank is an Iwagumi aquascape, combining fish with carefully crafted aquatic landscapes in miniature.