I have written an updated version of this post. Please click here.

So you’ve learned that goldfish cannot live in bowls and have to have a tank to be happy and live out their full lifespan.

[Edit: As you can see, I once believed this commonly repeated information. But once I learned the actual evidence shows otherwise, I ended up totally rewriting this post from a new perspective. Throughout this articleΒ  you will find the “enlightened” me arguing with my former tank size warrior self πŸ˜‰ ]

After all, they DO get bored and actually have a much longer memory than what most people assume.

The question arises: what size of tank do goldfish need?

Glass aquariums come in many shapes and sizes, and it can get overwhelming trying to figure out which one would be best for you and especially your fish.

Here is a helpful guide to selecting which tank size you should get.

What Goldfish Tank Size is Best for Your Fish?

Goldfish can’t live a normal life in small containers – they need their elbow room, too!

[Edit: Eek, no evidence of this, alas.]

Just as a person wouldn’t do well cramped up in a tiny closet their entire life, a goldfish won’t either.

[Edit: This is a false equivalency. You really can’t compare people to fish. And if they can’t do well in small homes, why did the oldest documented goldfish live for 45 years in a bowl just fine?]

Most people are surprised when they learn for the first time how big goldfish can get.

Fancy goldfish (the kind with the split or “double tail”) can easily reach 6 to 8 inches in length. INCHES.

Single tail varieties (such as the common, comet, shubunkin and wakin) will more than double that which is why they do great in a pond.

[Edit: Okay, so it’s true that goldfish CAN get really big under the right circumstances. But they can also self-regulate how big they get. It has to do with your husbandry.]

Do Goldfish Need a Big Tank?

Yes, absolutely!

[Edit: #facepalm]

If you want to promote healthy goldfish growth, your fish will need some space.

[Edit: Not necessarily, small goldfish can live long, healthy lives as well.]

You cannot have a tank that is too big when it comes to goldfish keeping. Β The bigger, the better… especially from your fish’s perspective.

Fancy goldfish need 10-20 gallons per fish.

[Edit: Note that these rules (while they could be a good starting point for beginners) vary widely from source to source and none are actually based on any scientific evidence.]

Slim-bodied breeds (like Common and Comet goldfish) need 40 gallons for the first fish, than 20 for each additional fish.

[Edit: So a single Common goldfish in a 39 gallon tank is now disqualified? See the problem with going by the gallons…]

This lets them grow to their full potential while making less work for you.

[Edit: If you want your fish to grow big, by all means follow the rule above. But without changing the water, you can still have a stunted fish! It’s not solely the tank size that controls the growth. There is no tank size “calculator” or formula you can use to accurately figure out how big of a tank you need; there are too many factors involved from aquarium to aquarium such as feeding, size of fish, filtration, plants, etc..]

Contrary to popular belief, larger tanks are actually less work than smaller tanks.

[Edit: It depends on how the tank is set up. I’ve since found small tanks can be easy to maintain with the right setup.]

You see, the water in a small tank gets dirty much faster than the water in a big tank, which means that you will need to do more frequent water changes with a small tank.

[Edit: Sometimes this can be the case with some setups, but again, proper filtration can really help. And even if that was the case, many people keep goldfish who can’t afford to have big setups and are willing to a little more work to change the water once or twice each week. Why shut them out?]

Goldfish are constantly producing a toxin called ammonia, which is very dangerous in small amounts and deadly in larger amounts.

Doing water changes is the only way to keep your tank from getting polluted, and water changes comprise the majority of the “grunt work” when it comes to goldfish keeping, as you will quickly discover if you haven’t already.

[Edit: No, actually doing water changes is NOT the only way to keep the water from getting polluted. That’s why we use filtration. Live plants also are amazing at purifying the water. But it’s true that water changes are grunt work πŸ˜› That’s why I’ve learned to focus on filtration so I don’t have to break my back!]

So please don’t be intimidated by the thought of having a larger tank, if you are used to small ones.

Your fish will thank you for it.

Tank Size Formula

For fancy goldfish, the general stocking rule of thumb is 10-20 gallons for each fish. Single-tailed fish need far more room, and at least 40 gallons for the first fish and 20 for each additional fish.

Many of you reading this may not be following this rule.

I certainly didn’t, when I was first learning how to keep goldfish.

As a goldfish enthusiast, I wanted almost every goldfish I saw, and bought almost every goldfish I wanted, and ended up owning 8 fancies at once. Β This resulted in disaster for my 10 gallon aquarium.

[Edit: This is the importance of adequately filtering a heavily stocked aquarium. I have since realized the problem isn’t the number of fish, it’s that the tank’s filter is not capable of processing all of the ammonia and nitrite. You probably can’t put 8 goldfish in a 10 gallon tank with your standard hang-on-back filter like I did and expect success without significant modifications.]

I couldn’t keep the ammonia under control and could never do enough water changes to keep up with the waste load that was being placed on my filter.

Worst of all, my fish were constantly sick and dying off.

[Edit: I was losing lots of fish to a contagious outbreak of dropsy at the time, which looking back I believe was actually unrelated to the water quality issue. Of course, ammonia doesn’t help things.]

But because I (selfishly) loved goldfish so much, I didn’t want to cut down my collecting habitsΒ to accommodate my tank size.

[Edit: You don’t have to do that if you supplement with filtration. I should have tossed in a sponge filter. But I was new to fish at the time.]

I had to learn my lesson the hard way when I lost ALL of my goldfish one by one until none were left, which was unfair to them and stressful and depressing for me.

Please, don’t make the same mistakes I did!

I know that sounds dramatic, but it really is a life or death issue for your fish.

Do things right the first time and follow the golden tank size rule!

[Edit: Sorry guys, I was wrong – following the rules doesn’t guarantee success. You can still have major water quality issues in big tanks, especially if uncycled.]

If you aren’t following the stocking rule, start.

Spend the money and get a larger tank for your fish if they are overstocked.

If you can’t afford it or don’t have room for a larger tank, consider taking back some of your fish or give them to someone who has the space. Β Sometimes you have to make a tough decision for the greater good of the collection.

*Sigh* the great mattersΒ we have to weigh as goldfish owners.

[Edit: Yikes, I feel so bad having given this advice. If you rehomed your fish because of what I said, please forgive me. :'( I was well-intentioned but misinformed. The only reason you need to rehome your new fish (unless there’s some other life circumstance going on or whatever) is if they’ve gotten too big from doing a lot of water changes and you don’t have enough space for them to swim comfortably.]Β 

Do Goldfish Grow to the Size of their Tank?

One myth about goldfish tank size has to do with whether or not a goldfish will stay small if kept in a small tank or bowl.

The truth is, it can…

But not how you think.

A goldfish kept in an undersized environment with constantly poor water quality often ends up stunted.

[Edit: Small tanks do not have to equate to poor water quality.]

This means the fish’s body stops growing while the organs don’t, and results in a deformed fish with a shortened lifespan.


So the fish may grow smaller, but only because it is being stunted and not because it is healthy.

[Edit: After extensive research, I have been unable to find any solid evidence that stunting is harmful to goldfish. To the contrary, the most recent 9 oldest goldfish in the world were all stunted. It is a widely circulated myth that the body stops growing while the organs do not.]

Some Helpful Stocking Tips:

  • Decide how many goldfish you want to own BEFORE you buy the tank. This will prevent you from being stuck with an undersized tank when you want to expand your collection. Goldfish tank size is very important to the overall health of your fish (and your workload).
  • If money is an issue when it comes to picking out a tank, try looking at garage sales or secondhand stores and get an aquarium used. This can help you save quite a bit and you can usually find a decently working tank that more often than not comes with equipment and accessories.
  • Stick to the plan. Β Once you have purchased your tank, don’t go crazy buying goldfish left and right. Β Don’t keep adding fish when your tank is totally full according to the golden stocking rule. Β If you just have to get more goldfish, upgrade your tank size along with them.
  • Here’s the best tip yet: buy the biggest tank you can afford! Β Really, I mean it! Β Save up, ask for a raise, mow lawns, do what you need to do to get a good size tank. Β If youΒ are crazy about goldfish, you will want a big family.

There you have it, folks.

Remember that your goldfish will thank you for giving them enough room and will reward you by growing big and strong and healthy.

Best of all, you will not be stressed out worrying about why your fish are always sick or fighting.

You are actually doing yourself a favor when you do them the favor of getting them a big home.

I’d like to hear your thoughts on goldfish tank size! Β Feel free to leave YOUR comment below.

The (Real) Conclusion

I’m not hiding anything folks…

As you can see from the above, I was once a big proponent of following stocking rules, years ago when I wrote the above post.

Most of the comments below also reflect my old viewpoint.

It’s all there for you to see.

But I’ve since learned that it’s not nearly as important as I once thought.

You’ll have to forgive me on this one.

We’re all learning on this journey of goldfish keeping.

That’s why I rewrote this entire article once I learned the truth and really dug into the nitty-gritty research to get answers.

Maybe you can do me a favor and just forget this one exists? πŸ˜‰

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