Today I’m going to shock you with a statement you would expect to hear from an uninformed pet store employee:

Goldfish don’t NEED a big tank.


“You’re joking, right?”

No, I really mean it.

After nearly 20 years of goldfish keeping, I’ve come to realize that all the hype about making sure your goldfish has to have an ocean to swim in is just that… HYPE.

And this is why:

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How Many Goldfish Can You Put in Your Tank?

goldfish in aquarium

Image Credit: panpilai paipa, Shutterstock

That, my friend, is the $64 million dollar question.

I’ve seen countless unsuspecting fishkeepers – new and experienced – get totally ripped to shreds (verbally) for posting a photo of their goldfish in “too small” of a tank (or even more horrifying, a bowl) online.

It’s like they might as well have posted:

“I’m an animal abuser who loves torturing goldfish.”

Why all the hate?

Because they’ve broken the unstated “RULES.”

The question is… exactly what rules?

You’ve probably heard any of the following stocking ratios for goldfish:


  • 1 inch of fish per gallon
  • 1 inch of fish per square foot of surface area
  • 10 gallons per fancy fish
  • 20 gallons per fancy fish
  • 20 gallons for the first fancy fish plus 10 gallons for each additional fancy fish
  • 30 gallons per fancy fish
  • 30 gallons per slim-bodied fish
  • 40 gallons per slim-bodied fish
  • 50 gallons per slim-bodied fish
  • 55 gallons per slim-bodied fish
  • Slim-bodies should only be kept in a pond

And these aren’t all.

(Well THAT makes things easy, right )

So you can see that there are all kinds of ideas floating around out there on exactly how many gallons of water a fancy or a slim-bodied fish needs…

… And precious little evidence to show why one is superior to the others.

No wonder people are confused about goldfish tank size!


When it comes down to it… those “rules” (none of which are in agreement) are just different guidelines people have made up over time, usually based on doing a 25% water change once every week or two and having a typical filter.

The problem?

None of them take into account that you might be able to have lots of plants, more water changes, carefully regulated feeding, or greater filtration capacities – things which can really offset having a smaller water volume. Every situation can vary drastically.

Do Goldfish Need a Big Tank?

It’s true:

Large goldfish (that have already reached a hefty size) do need a big tank to have enough room to swim properly.

But not if they’re still small.

Maybe you’re just starting with a little 2″ Comet you brought home from the pet store or a funfair.

Maybe you’re on a tight budget.

And maybe you don’t have a lot of room for a big tank.

Some people will go ballistic and tell you, “That fish is gonna get huge! Don’t you know it can’t live in such a small space?! You need to give your fish away, take it back to the store,” etc.

Not me.

And here’s why:

Goldfish CAN Grow to the Size of Their Tank

Yes, you’ve heard it right:

It’s a myth that goldfish are destined to outgrow smaller aquariums.


Goldfish produce a growth-inhibiting hormone (GIH) that builds up in the water.

When that water is changed all the time, the hormone is removed and the fish continues to grow.

A bigger tank helps to dilute this hormone, which is why goldfish tend to get really big in a big tank.


In a bowl or small tank, that hormone is very concentrated (unless lots of water changes are done all the time).

So it limits the fish’s growth.

And this is not the bad thing a lot of people think it is.

Many of the oldest goldfish in the world – the ones that have made it into their 30’s and 40’s, at the highest end of a goldfish’s lifespan – are what people would call quite undersized.

Really, they’re just environmentally stunted.

But their unusually long lives prove that they are healthy (an unhealthy, weak goldfish can’t live that long).

(Read more about why stunting isn’t a bad thing here).

Obviously, if your goldfish has already grown big, it’s too late for stunting to happen and it will need a bigger tank to have sufficient swimming space.

So young fish that haven’t grown much are better for keeping in a smaller space to avoid muscle atrophy problems.

That said:

Even some full-grown adult goldfish will never get to be monsters with tons of space, clean water, nutrition, and time.

These can be genetically stunted and can also do quite well in a smaller space.

Related Post: How Big Do Goldfish Get?

More Important than Goldfish Tank Size: Water Quality

dirty water

Credit: Chaikom, Shutterstock

Instead of getting obsessed about the tank size itself…

It’s far, FAR more important for a goldfish to be provided with clean water (in my opinion).

The way I see it:

A small, healthy fish is just as legitimate as a big healthy one.

As long as your fish is well-cared for and its needs are met, it’s a matter of preference – what works for your lifestyle and your objectives as a hobbyist.

I’m not going to say you’re wrong!

Tank Size FAQ

Successful goldfish keepers have kept fish for decades in a variety of containers supplied with good water quality.  Some of the oldest is a couple of commons in a 10 gallon (37 liters) tank won as a prize from a fair.

A full-grown fancy or slim-bodied goldfish (carassius auratus) can measure anywhere between 4.5 to 12 inches, depending primarily on genetic and environmental conditions.

The following factors influence the minimum size for a goldfish’s tank:

  • Size of the goldfish
  • Filtration type and amount
  • Water change schedule
  • Aquarium maintenance
  • Substrate
  • Varieties of live plants
  • Rate of feeding
  • Number of other fish

General recommendations or rules of thumb for a set number of gallons per fish vary widely from source to source.

While you may hear anything from “each goldfish requires a minimum of 20 gallons of water…”

… to “only a pond will do…”

Truth be told:

Goldfish don’t require a magic number of gallons to thrive if all of their needs are met.

But so many (contradicting) rules have been created!

And few – if any – are substantiated by any scientific fact.


First, understand that what it all boils down to is there are no set “rules” about tank size – there are too many other factors at play from fishkeeper to fishkeeper.


Listen carefully now:

Because water volume does NOT have a direct influence on the health of your goldfish (provided the fish can swim around enough to prevent muscle atrophy).

Water quality does.

Goldfish tank size can only play a direct role in the following:

  1. How frequently you have to go between water changes
  2. How big of a goldfish you can house (i.e. swimming room)
  3. And (potentially) aggression between more concentrated tank mates.


Some people really want rules or some kind of formula to feel comfortable.

That’s okay.

So long as you realize that following them doesn’t guarantee a safe environment for your fish (that’s in your hands).

(And so long as you don’t use your rules to club someone over the head who doesn’t follow them ? )


If you’re a beginner wanting a good starting point – a general suggestion – and you have the space and money – and you want them to grow to their full size, feel free to start out stocking your fancy goldfish tank at 10-20 gallons per fish.

Or 20 gallons, or 30, whatever you like.

A bigger tank means less work for you because the greater water volume dilutes toxins between water changes.

It also means your fish can get bigger comfortably.

This is more desirable if you are wanting your fish to get large.

As the hobbyist, you choose your goals.

If you want to keep a chunky 6-8″ fancy goldfish or even a 12″ slim-bodied fish, there’s no question that you’ll need more space for them (yes, many of them can get that big with the right conditions).

Some find their full-grown, foot-long single-tailed fish appreciate having as much as 55 or 80 gallons to themselves, others feel 30 or 40 gallons is enough.

If you’re looking in your tank and really feel like your fish needs more swimming space, by all means, go for it – pets are not fun to keep if you feel like yours is not happy.

As the owner, you know your fish best.

So more power to you ?


Some fish don’t seem to appreciate lots of space, others do, sometimes it just depends on the fish.

First and foremost, the priority is your water quality and the various factors in your tank.

(Your ammonia and nitrite always need to stay at 0.)

It also depends on the size of your fish to account for swimming space (a large fish will need more swimming room than a smaller one).

Did you get that?

It depends. 

Anyone who tells you you MUST have X number of fish per gallon or X number of gallons per fish is pulling these numbers out of thin air.

You can use rules for tank size if you want to.

But if you understand the much more important factors in fishkeeping, you don’t have to follow them to have a happy, healthy fish.

Considering there are so many different rules…

… you really end up making your own decision anyway based on what works for you. 🙂


Some people get confused at this point and think “Well if clean water is so important, and small homes require more water changes, that takes out the growth-inhibiting hormone so you will need a big tank anyway! So there is no way around this other than to cough up the cash for a mega-tank.”

People who ask this question are actually starting to get it ?

But they just haven’t realized the little-known secret that water changes are not the only way to maintain clean water.

See, there’s a difference between CLEAN water and FRESH water.

Clean water can be achieved through biological processes, the kinds used by your filter, and even live plants.

Fresh water is removing the current water and adding new water, which dilutes the growth-inhibiting hormones.

Let me help you with this:

If you’re having to use water changes to control the ammonia/nitrite output of your tank…

… your filtration is too weak.

You might need to consider adding more plants or a small sponge filter to stabilize things.

If you’re hitting 40ppm nitrates by Wednesday and your water change was last Saturday…

… it’s time to explore possibilities such as overfeeding, not enough plants, or not the right kind, or enough of the proper filter media.


Water changes have their place at times, but in the average tank they are a LOT of work and I prefer to create a more self-sustaining tank.

It’s less stressful on the fish and yourself.


If you want to find some scientific, study-backed, rock-solid evidence on small tanks being bad or big tanks being better (or vise versa) OR stunting being harmful or beneficial to goldfish, I’ve got bad news for you.

There isn’t any. 

(As far as I’ve been able to find anyway, and that’s after hours and hours of digging.)

But it’s not a tragedy to me we don’t have any, anyway.

A study on tank size only wouldn’t take into account how other factors can balance things.

Just consider all the endless variables:

  • Variety of goldfish
  • Substrate
  • Plant varieties & amount
  • Source of the goldfish
  • Health issues
  • Food amount, frequency, and type
  • Filtration type

All of these influence the well-being of the goldfish.

There are other ways to compensate for a smaller water volume.

You certainly can’t look at tank size as an isolated thing in real life among hobbyists.

Either way…

… Current research is silent. 

Maybe due to the number of years, it would take to prove an effect on longevity or the lack of funds.

For whatever reason:

This is something we as fishkeepers have to discover for ourselves by learning from other hobbyists and trying new things.

It’s part of the exciting journey of being a fishkeeper! ?


Don’t be fooled!

Other (rare) articles might try to pull in fish studies to back their position of goldfish needing X number of gallons.

But look closer and if you actually read these studies you’ll see the few that actually are talking about goldfish are not really talking about tank size at all, but rather their growth being harmed by things such as poor water quality, disease, or malnourishment!

They are being deceptive.

Studies on tank size in relation to other species of fish are IRRELEVANT.

The hormone that goldfish produce to regulate their growth is unique to them.

It’s their special superpower!


If you ever have someone trying to give you a hard time about the lack of studies supporting small aquaria for goldfish, ask them for studies supporting large ones for goldfish.

Read through whatever they give you (if they do) and you’ll see what I’m talking about.

You can also point them to the facts we do have…

… That the 9 oldest goldfish in the world all lived in small tanks.

I’ve heard some different arguments over the years against keeping goldfish in smaller aquaria.

(Again, “small” is pretty subjective for the most part – more on that later.)

This is one I’ve heard come up more than once.

“Small tanks are like keeping a child in a wardrobe with food and water or a dog in a crate without ever letting it out. Surviving is not thriving.”   


A tank can be too small.

I DO think it’s possible to keep a fish in too small of an aquarium – even if it has clean water.


If the fish isn’t able to swim properly, this could result in problems like muscle atrophy.

I’ve seen this happen – and I don’t advocate it.

(Because if the fish can’t turn around easily or swim properly, that’s a clear sign more space is needed – and that is not kind to the fish.)

So please be reasonable and give your fish enough room to swim, taking into account their size.

But to say that a fish that is 2″ can’t swim well in a 10-gallon tank usually comes back to the fact that that person assumes the fish WILL grow to 8″ long, which is not always the case (and does not have to be).


Back to the idea of “small…”

The “ideal environment” for a goldfish would be a lake full of 1,000’s gallons of water.


… Any indoor tank, be it a 55-gallon tank or a 5-gallon tank, is comparatively far, far less than that.

Neither one is “ideal” by nature’s definition.

In the big picture, one is not majorly different from the other.

Both are a much more limited and confined area than the animal would have in the wild (think wild dog vs. indoor dog kept in a house).

Realizing this will help us to understand we’re all in the same boat, to varying degrees.

Related Post: Are Small Tanks Cruel?

As to thriving:

That is a pretty subjective term.

But if longevity, appetite, health, and activity levels are any indication of thriving – the oldest goldfish in the world appear no worse off than fish in bigger tanks.

Just take a look at Fred & George (at a whopping 40  years old) here in action in their “undersized” tank.

No signs of distress whatsoever.

I’m amazed by how energetic they are despite their very old age!

Now tell me:

Where is the PROOF these goldfish are abused?

How does one determine that other than their own opinion – when to all appearances these fish are clearly happy and healthy?

But to continue with the silly analogy, now go and view a video of an abused dog kept in a crate its whole life or a child and you will see how different they behave and look.


Anyone with common sense (and an open mind) will realize that people who keep fish like this aren’t animal abusers. 

This brings me to my next point…

You may ask:

“How much swimming space is enough?”


I don’t have any kind of scientific answer to give you.

It probably depends on the size and activity level of the fish and ultimately as the owner, you would know better than anyone if your fish truly looks cramped.

The Japanese used a 3-4x the body length of the fish rule for horizontal swimming room when grooming the Tosakin.

Considering they were masters in goldfish breeding and keeping?

I’d say that’s not a bad rule of thumb.


Goldfish are not like people or children or dogs.

They swim everywhere they go.

This gives them a completely different perspective on space as they have much more freedom to move wherever they want to like they’re flying. ?

While a person or a dog relies on the square footage of the floor to be able to walk around…

… Fish don’t.

They can get all the exercise they need without nearly the room we or our land-dwelling pets would require.

Balancing Matters

Now, if you put 8 goldfish in a 10-gallon tank, it’s going to take a lot of effort to keep the water quality under control.

You’ll also have less room for error, and there’s a chance they might pick on each other more.

A word on aggression

Some people find that they have more problems with aggression when there are more goldfish in a smaller space.

Others don’t have a problem with this.

(And then there’s me, who has one fish in a 30-gallon tank all by herself who won’t tolerate anyone else, so go figure. 😛 )

It can depend on the fish’s individual personality.

My theory is there is less chance of trouble if all the fish are young – and introduced to the tank at the same time rather than one by one.

Siblings seem to get along better in my experience than randomly picked fish.

Sometimes aggression settles down once a “pecking order” is established.


If your tank is totally jam-packed with fish, it’s probably going to require a LOT of work to keep it safe for your fish.

So if you don’t want to spend your life slaving over them, a big tank can help free up your time.

In a more heavily stocked tank, overfeeding one of the biggest mistakes, followed by inadequate filtration and maintenance.


Clean water isn’t all there is to keeping your goldfish healthy.

Other aspects of their care are just as critical, such as proper nutrition and quarantine protocols.

Join the New Group

I get it:

Finding a safe, welcoming environment to learn about successfully keeping goldfish in smaller tanks is downright hard.

I’ve had many people tell me that they feel like they can’t learn more about goldfish keeping…

… Because every group they join starts bullying them. 

With that in mind:

I recently created a group SPECIFICALLY for people with “undersized” or “overstocked” aquaria that don’t go by the rules (which ones, anyway? 😛 ) – so they can feel comfortable and share their questions, stories, or tips with others.

It’s called Nano Goldfish Keepers.

(No harassing, bullying, or judgmental attitudes allowed.)

The term “nano” in the aquarium world generally means tanks under 20 gallons – but you can have a tank of any size to join though 🙂

So check it out and join if you like.

I’m very chatty on Facebook 🙂

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Wrapping it all Up

I’m sure not everyone is going to agree with me on this.

I also find that most people who criticize those who keep their fish in a small tank or bowl have ironically never owned a goldfish that made it past 10 years old, let alone 40.

But if you have any additional science-backed evidence that proves smaller tanks or stunting is harmful, I’d love to hear it in the comments below.

Have you learned something new?

Drop me a line!