How Big Do Goldfish Get? What’s the Average? And the Biggest EVER?!

Remember those stories you used to hear about people with pet alligators?

They’d get too big, so they’d flush the little dinosaurs down the toilet, and then they’d meet up with other flushed reptiles and breed in the sewers. Always seemed to be New York City in these stories.

The truth is that many pet owners do end up with critters that far exceed their expectations for growth. That’s why at the pet and aquarium shops they warn you about certain breeds that need big homes.

But how big do goldfish get?

How big do goldfish get written beside a big orange Ranchu goldfish with wen hood

They’re so incredibly inexpensive, lots of people start with them as a first fish. Surely they’ll stay nice and small and manageable, right?

Let’s take a look at this most famous, but widely misunderstood species and learn the truth. How big do goldfish get, and what size can they really grow to at the very biggest?

All That Glitters is Not Gold(fish)

First, lets clear up what is and what is not a goldfish. Those inch long things you win in a plastic bag at a fair? Those are goldfish. The big, shiny fish in backyard ponds and Chinese restaurant tanks? They are probably not.

There are many varieties of goldfish, each one with a distinguishing feature such as a large flowing tail, or enormous eyes. In fact, there are more than a hundred breeds to choose from, but they’re all selectively bred. Goldfish are a lot like dogs in this way.

Koi, a favourite with pond-keepers, are not goldfish, though they are related. They’re actually a subspecies of carp, bred for colour and shape since the 1820s. Left to their own devices, however, they’ll look like ordinary carp again within a few generations.

Goldfish were also bred from carp, (Prussian carp, if you really want to know) but this began close to a thousand years ago. Since then, they have been established as an entirely separate species. Given their ancestry, however, it’s not impossible to imagine they have the potential to grow quite large.

The Biggest Fish In the Pond

Koi and carp can both grow to enormous sizes.

The largest koi on record weighed a whopping 90 pounds. But that’s nothing compared to a carp landed by a British angler in Thailand in 2016. This genuine monster was a backbreaking 222 pounds. Concerned you might need a swimming pool-sized tank to contain your goldie?

Don’t be. The world-record for a common goldfish is just over 5 pounds. Not insignificant, but not the premise for a straight-to-video horror movie, either.

Will My Goldfish Get Too Big For the Tank?

There is a common misconception that a goldfish will grow to fit its environment. This is true in a sense, but not in the way people think; putting a common goldfish in a mega-tank won’t cause it to grow to the size of a tuna.

What can happen is living in cramped or unsanitary conditions may stunt a goldfish’s growth. So, rather than growing to the size of the environment, the environment may actually limit the fish’s growth.

You can also limit its lifespan, too. Living in too small an environment stresses a fish and leaves it susceptible to disease. It can cause any number of physical ailments. Sadly, many people think they only live a few years at most because historically they’ve been kept in bowls and tiny aquariums.

A well cared for goldfish can live for two or three decades (with several documented cases of living past 40) so be sure you’re ready to make the commitment.

A Bit of Nature, A Bit of Nurture

How large your goldfish will grow is largely dependent on its genetics. Just like with humans, if mom is big and dad is big, baby will probably be big, too. The problem is, if you don’t know who the parents are (which you probably won’t if you didn’t breed the fish yourself), there’s no real way to tell what your goldie’s potential is.

If you want your fish to reach its maximum size, there are a few things you can do:

  • Provide a large aquarium (at least 15 gallons for each fish) with plenty of length
  • Keep the water clean
  • Use nutrient-rich food
  • Do NOT keep goldfish in a bowl

How big do Goldfish Get? How Big is Mine Likely to Grow?

It’s hard to say, but most common goldfish have the potential to reach up to 10 inches in length under ideal conditions. Fancy varieties tend to top out at 8 inches. Those bred for pond life may grow even larger, perhaps up to 18 inches long.

Of course there are no guarantees, but it’s important to plan for the best-possible scenario to ensure your goldfish is healthy and happy.

Want to see what your goldfish could end up like? Have a look at this video of a fish won at a carnival that’s now about 8 inches in length. That’s a seriously big specimen!


Final Thoughts

Were you surprised to learn that little Goldie has the potential to turn into Gold-zilla? Makes you want to treat them with more respect!

Goldfish are a wonderful choice for beginners, and for experienced aquarists. Just make sure you know exactly what you’re getting before you bring one home.

That’s all we have to say on this subject, but there’s plenty more to be learned! Please browse the site and see what more burning questions we can answer for you.

Thanks for sharing your day with us, and happy fish keeping!

Wendy Kathryn

Hi, I'm Wendy, the owner and creator of this website, an experienced fish keeper and avid student of the art since 2010. My aim is to help beginners avoid the many possible mistakes when getting started in this wonderful hobby.