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Do Goldfish Grow to the Size of their Tank? 3 Myths Debunked

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By Lindsey Stanton

muliple sized fish tanks

Goldfish have been, are, and continue to be one of the most misunderstood pets, but not in the way you might think.

Goldfish can and have grown to the size of their environment by self-limiting their growth (talking about bowls and small tanks here), and they can thrive at that smaller size!

It’s all because of something called STUNTING.


What is Stunting?

goldfish bowl
Image Credit: LUIS PADILLA-Fotografia, Shutterstock

Stunting can be environmental or genetic. This ability is one of the major reasons why goldfish have been the most popular aquarium fish worldwide – because they don’t have to be given massive areas to live in.

It makes them accessible to the hobbyist who doesn’t have much money or a lot of space for a big tank. The evidence shows that small, stunted goldfish are actually more resilient and longer-lived than those that are grown to be large sizes.

Goldfish CAN indeed get huge. But only if conditions favor that, and if not, they are able to inhibit their own growth, which is pretty amazing. The concentration of the growth-inhibiting hormone somatostatin limits the fish’s growth in a smaller environment (without large frequent water changes).

This keeps a fish that can grow 12 inches in a big space around 3−5 inches in a smaller space. So far, I have yet to see any evidence to support the notion that the organs of the fish keep growing when the fish does not. Real-life examples indicate stunted goldfish actually have a longer lifespan than average.

On the flip side:

And some goldfish never get to be a tenth of what is considered full size no matter HOW much room and fresh water they get because it comes down to genetics. I’ve seen many people go out and buy a huge tank expecting their fish to become a whale…

… And you know what? It NEVER happens. They feed them well, they change the water, they give them lots of room and nada.

Perpetually small fish.

Image Credit: New Africa, Shutterstock

Why? A goldfish does most of its growing in the first year of its life, and if it’s going to be a big one, that’s usually when the growth happens. Much of it comes down to genetics. Those fish that don’t grow much are genetically stunted.

Whether a fish is environmentally or genetically stunted, there is no evidence that this has a negative impact on their health or lifespan – quite the contrary. The world’s oldest goldfish are often stunts kept in small aquaria.

Read more: Stunted Goldfish Growth: How it Happens (and is it Harmful)

1. Groundless Tank Size Requirements Used to Create Guilt – And Make a Profit

Image Credit: chaikom, Shutterstock

I’m letting loose on something that really bothers me. Something that happens especially at pet stores, online forums, and groups. And it’s needlessly discouraging would-be goldfish owners from joining the hobby – as well as leaving countless goldfish who would be rescued from horrible conditions to be doomed to a sad fate.

You might have heard something like this before at a pet store: “Hi, I’m here to buy a feeder goldfish for a pet for my 5-year-old.”

“Oh no, don’t you know that all goldfish grow huge? And those little feeder fish get to be over a foot long! They have to have a big tank. 40 gallons minimum, ideally a pond. If not they will be stunted and harmed for life. Right this way ma’am, I’ll show you our 40 gallon breeder tanks. You’ll need a filter that turns over 10x the water volume for that, and for water changes this hose kit that connects to your sink will need to remove 50% of the water weekly.”

“So, you’re telling me this $0.14 feeder fish needs over $100 worth of housing equipment to take care of if – not including the water bill?” “Yep that’s right.

This results in 2 common outcomes:

  1. Some people see how ridiculous this is and go somewhere else. But the sad thing is those folks never learn what goldfish REALLY need to stay happy and healthy, and if they try to keep fish their own way many times (not all the time but many times), they fail and give up – all because they were never told the truth. So they never buy any more goldfish.
  2. Others feel guilted into thinking if they can’t afford or don’t have space for a huge aquarium, they’ll be an animal abuser, and it’s time to head over to the betta fish section.

The bottom line? Poor feeder goldfish that have battled to stay alive this far and could have been rescued remain at the store until some bearded dragon owner brings them home just to be eaten alive.

And on the flip side, many folks never get to know the joy of keeping goldfish. People who could have gone on to explore more of the exotic varieties miss out on a whole world and never realize how great, easy, relatively inexpensive pets they can be.

You just have to know what their REAL needs are – and how to achieve them.

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. 

Instead, teach them how to keep their little fish healthy through good water quality and proper feeding habits (even if they never achieve whale status).

Related Post: Why Goldfish Tank Size Isn’t as Important as You Think

Sadly, using false information to create guilt and make massive up-sells on expensive products is dealing a heavy blow to the hobby as it excludes those with smaller incomes or living areas.

But this practice is nothing new. Goldfish bowls have been slandered by aquarium manufacturers since the late 1800s (coincidence?). But the oldest documented goldfish in recent history lived in one just fine for 43 years.

2. Pet Store Feeder Goldfish Left to Die Without Homes

dead goldfish
Image Credit: Zay Nyi Nyi, Shutterstock

Many of these people truly believe and convince others to believe that it’s better for a fish to end up destroyed than put in anything less than x gallons of water in a forever home. Pet stores claim their crowded conditions are temporary.

But have you ever looked in those tanks and noticed how many dead fish are there? You don’t even see all the casualties, as employees have to remove the dead fish daily – sometimes PILES of them.

As if it couldn’t get any worse, add in the rampant disease problems in these setups contaminating most of them. It’s like a prison camp for fish! How many fish die slow, miserable deaths before ever making it to their final digested destination?

Ever seen what can happen to fish who are put in the same tank as a sharp-toothed predator? This shows how much pet stores REALLY care about these fish and creates a lot of confusion. If you don’t buy the expensive gear, they’ll just sell that fish to a more “caring pet owner”—a carnivorous pet, that is.

While turning down owners who would do their best to give their fish a loving home with all their needs met, believing the fish is going to outgrow the small tank and that all goldfish will get big (which is a lame reason, in my opinion, as even if it did outgrow the tank due to lots of water changes, the owners can just upgrade).

But I don’t blame the pet stores entirely. No, because well-meaning but misguided fish owners have helped to pour gasoline on this fire.

  • They get angry when they see a small tank advertised with goldfish in it at the pet store, then proceed to try to “educate” employees or complain to managers using information pulled up through a quick internet search.
  • They verbally thrash new fish owners who have their fish in anything under x number of gallons.
  • They fight against fish being given away at fairs and make petitions against small homes for goldfish

What they don’t realize is they are actually killing more fish than they are helping by causing a lot of false panic and false guilt. Instead, focusing on the real threats to goldfish health would get us much farther.

3. Untreated Disease a REAL Problem

bloated dropsy goldfish
Image Credit: Wari_Nuchczy, Shutterstock

What pet stores and these internet warriors DON’T tell you is what these fish need more important than a fancy fish mansion is health care. Because frankly, most workers have no idea how parasite-ridden the majority of these poor fish actually are.

In fact: Training veterinarians use them to study fish disease, they’re so contaminated! People guilt-trip unsuspecting owners-to-be into giving them what they consider to be “proper care” when, in fact, many of those things aren’t necessary at all.

They don’t inform them that their pet is actually likely to die from a disease they got in those cramped, stressful conditions at those facilities, where NO proper quarantine was performed to ensure your fish are healthy when you buy them.

And that goes for all of their fish, not just the throwaway feeders. They’re selling you sick fish! If people are going to spend money on these fish, shouldn’t it be (first and foremost) for something they really need to survive, such as quarantine treatments?

Who cares if they have an ocean to swim in with a high-tech filter and colorful decorations if they only end up dying a month or two later from one of many parasitic infections?

Related Post:

wave tropical divider


We need to focus on the real problems here if we want to help fish.

  • Stop worrying about fish having x number of gallons of water and emphasize clean water over more water. More dirty water never helped any fish.
  • Start emphasizing the importance of proper quarantine procedures for all fish that do not come from trusted breeders.

This is my take. Maybe you have a different opinion. If so, you are invited to share it below.

Featured Image Credit: MOUii, Shutterstock

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