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

Okay, 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?

To me it seems there a lot of rumors floating around without much evidence behind them – as well as some downright myths.

Keep reading to learn more!

Is it Bad to Stunt the Growth of a Goldfish?

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.

For example:

One big fear about stunting is that “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.


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.

Until I actually see any evidence, I believe this is a total myth.

Another rumor goes that a stunted goldfish will have structural deformities that can harm its health, like a bent back, bigger eyes or crooked mouth.

Again, no evidence exists for that as far as I can tell – 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. There are other factors that 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 a pattern that proves stunting was the common thread that caused the problem

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 (I mean, if you’re going to get mad about that, what about the Telescope or Bubble eye!)

Or, “a stunted goldfish won’t live long.”

Now this one seems pretty odd to me, because out of the recent 7 oldest goldfish in the world – just about all of them were not even half the size they would normally grow.

I.e., a couple of fair fish in a 10 gallon tank (yes, slim-bodied fish).

But you know what DOES cause a shorter lifespan?

Fast growth rates from higher temperatures (increased metabolism), lots of space and ample food.

This is commonplace with the majority of goldfish on the market – 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 livespans don’t seem affected.

There are other factors that influence lifespan, but so far I can’t find evidence that stunting is one of them.


I’m not going to say either one is bad!  It depends on your space, finances and goals as a hobbyist.

It seems stunted goldfish can live long, healthy lives.

What about “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.

The bottom line?

Until there are some facts presented behind the so-called negative effects of stunting, I consider them nothing more than myths.

What Causes a Stunted Goldfish?

I’m going to shock you by saying this:

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.

1. Not doing frequent water changes.

You’re probably wondering:

“What does THAT have to do with stunting?”

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.

Water changes remove and dilute that, allowing the fish to get bigger.

But it’s true:

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

2. Genetics

Some people are surprised when they hear me say this:

“Yeah, your goldfish might not ever grow much – no matter how much food and clean water it gets.”


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.

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, chances are you don’t have much control over this factor.

4. Nitrates

There is evidence that higher nitrates can cause fish to grow slowly, and/or have a limiting effect on fish growth.

Typically more water changes leads 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.

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

Wrapping it Up

I hope you found this article informative.

Stunting can be avoided, but as the 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?

Got any evidence that shows I’m in the wrong? I’m totally open to discussion on this.

Leave your comment below!

5 100% from 4 ratings
Rating 5 100%


  1. Vanda Krutnik March 21, 2019 at 2:48 pm - Reply

    Great info – thanks!

    Rating: 5
    • Pure Goldfish
      Pure Goldfish March 21, 2019 at 4:52 pm - Reply

      You’re welcome, thanks for stopping by Vanda!

  2. Maora Oakie March 23, 2019 at 6:34 am - Reply

    With this in mind, would it be alright to keep a black moor in a 5-10 gallon tank?

    Thanks for the info btw. I’m glad to know that goldfish don’t HAVE to be super huge. It’s really the only factor stopping me from buying one

    Rating: 5
    • Pure Goldfish
      Pure Goldfish March 23, 2019 at 5:25 pm - Reply

      Provided you get him when he’s small. He may outgrow the tank if you do lots of water changes rather than relying on filtration.

  3. Elaine March 26, 2019 at 7:19 pm - Reply

    You put my mind at ease. I was told that the organs would grow too big for the body and I envisioned a slow painful death. Thank you for straightening that out.

    Rating: 5
    • Pure Goldfish
      Pure Goldfish March 26, 2019 at 10:41 pm - Reply

      You’re quite welcome, Elaine!

  4. Karter April 19, 2019 at 1:00 am - Reply

    With all of that being said, would it be logical to assume that (if a smaller goldfish is the goal) adding large amounts of goldfish friendly aquarium plants would reduce nitrate levels and thus reduce (obviously not eliminate) the need for large water changes? That way growth inhibiting hormones do not become too diluted.

    Rating: 5
    • Pure Goldfish
      Pure Goldfish April 19, 2019 at 3:37 pm - Reply

      You got it, Karter!

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