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Stunted Goldfish Growth: How It Happens (and Is It Harmful?)

Lindsey Stanton Profile Picture

By Lindsey Stanton

small goldfish in goldfish bowl

I’ll preface this by saying this is a controversial topic with heated opinions on both sides. But I really think it’s good for us to look at this objectively. So, today I want to talk about stunting as it relates to the growth of a goldfish.

How does it REALLY happen? More importantly… Is it bad for them? It seems there are a lot of rumors floating around without much evidence behind them – as well as some downright myths. Keep reading to learn more!

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What is Stunting, Anyway?

ryukin goldfish in tank
Image Credit: Moo teaforthree, Shutterstock

Stunting is what causes a goldfish not to grow as large as it would otherwise. While a young common goldfish could reach 12″ in length given specific circumstances…… That SAME fish – stunted – might only grow 4 or 5″ and stay there under different ones.

Many factors can cause stunting, not all of which are safe for the fish – or even in our control (more on this later.) Goldfish are unique in their ability to self-regulate their growth. How? It comes down to a substance that all goldfish produce called somatostatin. 

That’s the growth-inhibiting hormone (sometimes abbreviated to GIH) goldfish secrete that suppresses the growth of other goldfish in their environment and the hormone that can actually suppress the fish itself if it builds up in the water. It’s also referred to as “growth regulatory hormone.”

Not all species of fish have this ability. In smaller environments or those where the water is not changed as often, the hormone is more concentrated. This limits the growth of the fish. Now we’re going to get into the controversial part, so hold onto your hats.

Is it Bad to Stunt the Growth of a Goldfish? 5 Common Myths

goldfish in bowl
Image Credit: DJ Srki, Shutterstock

There are many opinions out there on stunting (in goldfish, specifically, I’m not talking about other species), but few – if any – seem to be actually substantiated by fact. There honestly isn’t a lot of research available on the topic.

I’ve linked some studies at the bottom, but I’ll be 100% transparent and say NONE of them are about the effects of stunting on goldfish specifically. Because it seems the scientifically test-verified evidence doesn’t exist for it. What it all boils down to?

There is a clear lack of formal documented trials and results as it relates to this special animal. So I believe this has left us with a lot of unfounded myths and fears. And a lot of people say a lot of things that aren’t backed up by any relevant research – while actually contradicting what’s observable to us in real life.

For example, one of the biggest fears about stunting is that

1. “It causes the fish’s body to stop growing, while the organs do not.”

And POOF – one day like an atom bomb, your goldfish will just explode. Really? So far, I have yet to see any scientific or anecdotal evidence of this happening in the thousands of goldfish keepers I’ve talked to.

Logically – if that were true – then all these old stunted goldfish we see today should have grossly swollen bodies trying to contain organs intended for a fish 3x their size. But, clearly that’s not the case.

Stunted goldfish DON’T look swollen or distended any more than a “normal” goldfish. Until I actually see any evidence otherwise, I believe this is a total myth.

Another rumor goes that

Image Credit: Hans, Pixabay

2. “A stunted goldfish will have structural deformities that can harm its health, like a bent back or crooked mouth.”

Again, no evidence exists for that as far as I have been able to find, but there ARE plenty of studies that show malnutrition can directly cause skeletal deformities and poor health. So when you find those stories floating around on the internet, remember that:

  1. Other factors could be the cause of the fish’s problem (one stunted goldfish may have a bent back, but it could very well be that it was lacking vitamin C in its diet.
  2. Not enough fish are considered on a broader scale to find any kind of pattern that proves stunting was the common thread that caused the problem. (In other words, it takes a bigger group of fish to detect common patterns to be able to form a conclusion – you can’t base it off of just one fish. In my extensive research, I have been unable to find any studies – small or large scale – on the effects of stunting on goldfish for one way or another.)

Some slightly different proportions, such as larger eyes, seem to be often seen with stunts, but that doesn’t seem to be harmful to the fish.

Credit: Japan’s Fireworks, Shutterstock

3. “A stunted goldfish won’t live long.”

Now, this one seems pretty odd to me because out of the recent 9 oldest goldfish in the world – ALL of them were not even half the size they would normally grow.

In a small tank without frequent water changes, the fish’s growth is barely noticeable. This slower growth is actually linked to a longer life! But you know what does cause a shorter lifespan?

Fast growth rates can be achieved with higher temperatures (increased metabolism), lots of space, and ample food. Knowledgeable breeders will attest to this.

This is commonplace with the majority of goldfish on the market who are grown to size as quickly as possible – and these fish rarely make it past 10 years!

Finally, let’s look at the bonsai koi. These fish never get anywhere close to the size they normally would in a big pond, but with the right care, their lifespans don’t seem to be affected. Other factors influence lifespan, but so far, I can’t find any legitimate evidence whatsoever that stunting is one of them.

Which is better, a stunted fish or a non-stunted one? I’m not going to say either one is bad! It truly depends on your space, finances, and goals as a hobbyist. Stunted goldfish can live long, healthy lives – even more than normal.

Instead of having a shorter life, it seems the little, slowly-grown stunted goldfish consistently outlive the big ones!

This is also true of many other animals, including dogs:

“The strange quirk is that while bigger species of mammals live longer than smaller ones, large size is not an advantage if we confine our analysis to one species at a time. Within any single species we find that the trend is reversed, and it is the smaller animals that have the longer lives. This is certainly the case in dogs. Data suggests that this is even true in humans, since larger people tend to have shorter life spans.” {7}

wakin goldfish
Image Credit: JuanCarlosPalauDiaz, Shutterstock

If you're new to the world of goldfish or are an experienced goldfish keeper that loves to learn more, we recommend you check out our best-selling book, The Truth About Goldfish, on Amazon.

From diagnosing illnesses and providing correct treatments to ensuring your goldies are happy with their setup and your maintenance, this book brings our blog to life in color and will help you to be the best goldfishkeeper you can be. 

4. “Stunted goldfish are weaker and have a propensity to disease.”

Again, I have seen zero evidence to prove this. If it’s true, such fish wouldn’t live long.  At the risk of being redundant, most of the oldest goldfish are stunts. Stunted Bob even survived a tumor removal surgery at 20 years old.

But poor nutrition HAS been linked to a lower immune system. {4} The bottom line? Until there are some facts presented behind the so-called negative effects of stunting, I consider them nothing more than myths.

Many of the nation’s most respected goldfish breeders have similar opinions.

Image Credit: antoni halim, Shutterstock

5. “Stunting is always permanent.”

There’s a difference between RUNTS and STUNTS. Runts are genetically stunted. Their growth is not determined by their environment. No matter how much food and fresh water they have, they will never really grow much.

On the other hand, there are stunts that are environmentally stunted. Dave Mandlay is an experienced goldfish breeder who supplies fish to several big box stores in the US and Canada.

He weighs in on stunts:

“They [environmentally stunted goldfish] will resume growth and breed when spaced out in ponds or aquariums. They are not stunted from a genetic point or hindered in future development, just kind of ‘Held Back.'” – Dave Mandlay, alias bluebelly, quoted from https://goldfishkeepers.com/community/threads/growing-out-runts.5039/

Because it points to stunting being a survival mechanism when times do not allow for growth that can actually REVERSE in times when they do. So if you’re worried about permanently limiting your fish’s growth while you wait to upgrade your tank, it doesn’t seem like you have anything to worry about.

On the other hand, if you’re someone who doesn’t want to upgrade to a bigger tank, a runt may be a better option if you don’t want to worry about GIH export during water changes.

I’ve talked to Mr. Mandlay personally, and he has informed me that emersed plants grown in setups containing fish remove GIH from the water. This is why goldfish and koi can still get HUGE even in crowded aquaponics setups with little water changes.

Goldfish in aquarium with green plants
Image Credit: Skumer, Shutterstock

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What Factors Cause a Stunted Goldfish?

It’s actually NOT small tanks that cause stunting.Nope, the size of the fish’s enclosure does not control the fish’s growth directly. As proof, a single goldfish can become stunted in a 100-gallon aquarium!

How is this possible? There are actually multiple causes. I will talk about that more in the post below.

Related Post: Goldfish Growth: Everything You Want to Know

1. Not doing frequent water changes.

You’re probably wondering, “what does THAT have to do with stunting?” Water changes remove and dilute growth inhibiting hormones, allowing the fish to get bigger.

It is more common for goldfish in smaller tanks or bowls to become stunted, but the reason is that growth-inhibiting substances build up FASTER in them than in a more diluted large aquarium.

dirty water
Credit: Chaikom, Shutterstock

2. Genetics

“Yeah, your goldfish might not ever grow much – no matter how much food and clean water it gets.” Why? The common idea is that ALL goldfish WILL grow to be 6-8″ if they are a fancy and 12″+ if they are slim-bodied as long as they are given “proper care.”

But what they don’t know is in every spawn of goldfish, there are going to be runts. Some will never get to be more than a tenth of the size of their biggest sibling.

Fancy goldfish, especially some of the generally more petite varieties, may stop growing long before when you think they should end, no matter what you do. This could be just plain genetics.

Related Post: How Big do Goldfish Get?

3. Early-Years Husbandry

A goldfish does most of its growing in the first year of its life. Depending on how it was cared for during that time, it may or may not continue to grow to be much bigger. Unless you breed goldfish yourself or acquire very young goldfish to start with, chances are you don’t have much control over this factor.

Image Credit: chaikom, Shutterstock

4. Nitrates

There is some evidence that higher nitrates can cause fish to grow slowly and have a limiting effect on fish growth. {5} Typically, more water changes lead to lower nitrates.

This means a greater chance for growth as you are removing both nitrates and hormones from the water during a water change. Nitrates are totally within the hobbyist’s control.

5. Malnutrition

Finally, let’s talk about malnutrition. This cause of stunting is probably the only really bad one because it directly harms the health of the goldfish, leading to a compromised immune system and propensity to disease. {6}

Vitamin and mineral deficiencies can cause the growth of the fish to be stunted in addition to causing other problems.

Credit: epicioci, Pixabay

If you're new to the world of goldfish or are an experienced goldfish keeper that loves to learn more, we recommend you check out our best-selling book, The Truth About Goldfish, on Amazon.

From diagnosing illnesses and providing correct treatments to ensuring your goldies are happy with their setup and your maintenance, this book brings our blog to life in color and will help you to be the best goldfishkeeper you can be. 

Join the New Group

I’ve heard from many people who want to learn more about this topic. Or maybe explore keeping goldfish in smaller aquaria. But going against the grain of what most people think is acceptable.

So I decided to create a Facebook group where people can feel comfortable and welcome talking about these things. I call it Nano Goldfish Keepers (“nano” means tanks under 20 gallons when it comes to aquariums).

You’re welcome to check it out – I’m very active there.

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Wrapping It Up

I hope you found this article informative. Stunting can be avoided, but as a hobbyist, you choose your goals. Not every goldfish needs to turn into a monster! So what do you think? Have you ever had a stunted goldfish?


1. Somatostatin inhibition of growth hormone release in goldfish: possible targets of intracellular mechanisms of action. https://www.ncbi.nlm.nih.gov/pubmed/9405121

2. L-ascorbyl-2-sulfate alleviates Atlantic salmon scurvy https://www.ncbi.nlm.nih.gov/pubmed/8073052

3. Nutritional Diseases of Fish in Aquaculture and Their Management: A Review https://www.actascientific.com/ASPS/pdf/ASPS-02-0171.pdf

4. The Role of Somatostatins in the Regulation of Growth in Fish https://link.springer.com/article/10.1023/B:FISH.0000032727.75493.e8

5. Chronic exposure to nitrate significantly reduces growth and affects the health status of juvenile Nile tilapia (Oreochromis niloticus L.) in recirculating aquaculture systems https://onlinelibrary.wiley.com/doi/abs/10.1111/are.13174

6. Nutrition and Fish Health http://www.glfc.org/pubs/SpecialPubs/sp83_2/pdf/chap8.pdf

7. Why Do Large Dogs Have Shorter Life Spans Than Small Dogs? Recent Data Explains why Larger Dogs Age More Quickly https://www.psychologytoday.com/us/blog/canine-corner/201701/why-do-large-dogs-have-shorter-life-spans-small-dogs

Featured Image Credit: New Africa, Shutterstock

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