Goldfish Tank Size Guide

Choosing the right goldfish tank size is absolutely critical if your pets are to thrive, grow properly and live a long, healthy life.

Read this guide to learn why goldfish tank size is so important and how to work out what size goldfish tank you need.

Is goldfish tank size important?

Yes! Goldfish tank size is absolutely crucial for several reasons:

Goldfish Size

That tiny, inch-long goldfish you bought from your local pet store won’t stay that size for long!

Goldfish are distantly related to a species of wild Prussian carp. The carp can grow to reach a length of 18 inches and weigh over 6 pounds, so it’s no surprise that your goldfish can grow to be pretty sizable.

For example, a common, slim-bodied goldfish, such as a Comet, can reach over a foot in length, while an egg-shaped Fancy goldfish can grow to measure 8 inches long or even more. So, you can see that a small fish tank will not be at all suitable for a goldfish!

Stunted Growth

Contrary to the popular urban myth, goldfish don’t simply grow to fit their container and then stop!

Your goldfish will keep on growing, regardless of the size of his tank. 

Unfortunately, that can result in physical deformities and stunted growth. So, if you want your goldfish to grow properly, stay healthy and thrive, you must ensure it has a tank of the correct size.

Fish Tank Cleanliness

Goldfish are one of the dirtiest pet fish you can get!

Did you know that your goldfish doesn’t have a stomach like you and I? Everything your fish eats passes straight through your pet’s digestive system, with the nutrients he needs being extracted along the way. The waste products that are left pass out of the goldfish into the tank water.

Goldfish eat a lot! In fact, between feeds your goldfish spends most of his day foraging in the tank. 

Any scraps of leftover food, algae, and tender plants make great snacks for a peckish fish. And all that food generates even more waste!

Dangerous Water!

All that waste gradually decomposes, producing a highly toxic chemical called ammonia. Ammonia is poisonous to your goldfish. 

If the ammonia is not removed by your filtration system and through partial water changes, it is processed by bacteria that convert the ammonia to nitrites. Nitrites are also extremely dangerous for your goldfish. 

If the water becomes too polluted with ammonia and nitrites, your goldfish will get sick and eventually die!

If your goldfish tank is too small, the water will rapidly become loaded with toxic chemicals produced by the fish’s waste. Now, you could carry out partial water changes, rinse the filter media, and clean the tank more frequently. However, that’s a lot of work. 

So, it makes sense for both you and your fish to house your pets in a larger tank. That way, the fish waste doesn’t accumulate as quickly, and the environment stays cleaner and healthier for the goldfish.

How many goldfish can I keep in a tank?

You need to keep at least two fish in a tank, ideally more. To do that comfortably, you need a large enough tank to accommodate them. 

Goldfish are gregarious creatures that do best when kept in the company of others of their species. It’s actually pretty cruel to keep one solitary goldfish on his own. In fact, in 2008, a law was passed in Switzerland making it illegal to keep one goldfish alone.

Goldfish Tank Size Requirements

The size of the fish tank you need depends on the type of goldfish you want to keep.

Comet and Common Goldfish

Since Common goldfish, Shubunkins, and Comets can grow to over a foot in length, we recommend a fish tank of at least 30 gallons and 4 feet long, or ideally bigger than that. The tank should have a tightly fitting lid, as these fish can jump if they feel alarmed.

If you want to keep more than one slim-bodied goldfish in your tank, you should add an extra 12 gallons per additional fish

So, for two Common goldfish, you need a 42-gallon tank. That’s 30 gallons for the first goldfish, plus an extra 12 gallons for the second fish. 

If the fish feel cramped, they will become stressed, which can lead to health and developmental problems. These large, fast-swimming fish really need more space than is available in a tank and are really much better suited to life in a large garden pond. 

Fancy Goldfish

Fancy goldfish should be kept in a tank of at least 20 gallons, measuring a minimum of 3 feet long

Fancies are typically clumsy swimmers, wobbling around the tank in a very comical fashion. If the tank is too deep, your fish will struggle to reach the surface to feed, so a shallow, rectangular tank is best. 

Keep the main swimming area clear of decorations and heavy planting so that the fish won’t crash into things! 

Add an extra 10 gallons for each extra Fancy goldfish you want to keep. 

You need a 30-gallon tank for two Fancy goldfish. That’s 20 gallons for the first fish, and then an extra 10 gallons for the second. 

If you want to keep more than two goldfish, you’ll use the same calculation, but the tank size you’ll need would be pretty huge, and that’s not practical for most people.

Mix and Match?

Although all types of goldfish are peaceful creatures that can be kept together safely, we don’t recommend mixing Fancies with slim-bodied Comets and the like.

How so?

Well, it’s really a safety thing. Fancy goldfish are slow, cumbersome swimmers, and some varieties, such as Bubble Eye goldfish, have impaired vision. 

Slim-bodied goldfish in comparison, are fast, agile swimmers that will simply barge past the slower Fancies, especially at feeding time. That can result in injuries to your fish, and the slower fish might even go hungry.

So, we recommend that you stick to keeping either Fancy goldfish or slim-bodied goldfish rather than mixing them.

How to tell if your goldfish needs a bigger tank

Your goldfish should have plenty of space in which to swim and forage, and the water in the tank should be clean and healthy for your pets.

There are some signs to look out for that might indicate your goldfish needs a bigger tank:

  • The fish bump into each other or collide with decorations while swimming.
  • The goldfish become lethargic and inactive within a day or so of you cleaning the tank.
  • Your goldfish stop eating and foraging for food.
  • Your fish develop ulcers, ragged fins, or reddened skin due to bacterial infections.

The last three of those signs are directly related to poor water conditions and stress. 

If the water is polluted, which often happens in overcrowded tanks that are too small, the fish become stressed. 

When fish are stressed, their immune system is compromised. That means the fish are less able to fight off disease and are more likely to become sick.

Consequences of a small fish tank

As explained above, if your fish are kept in a fish tank that’s too small, the water quality will suffer. No matter how often you carry out partial water changes and how diligently you maintain your filter system, your fish will quickly show signs of nitrate poisoning.

Overcrowding causes stress, which damages the creatures’ immune system, leaving your fish open to attack by parasites and bacterial disease.

In extreme cases, your fish won’t grow properly, leading to stunted growth and physical deformities.

So, you can see how important it is that you keep your goldfish in a large enough tank.

How to calculate goldfish tank size

But how do you know if your current goldfish tank is the correct size for your fish?

Fortunately, it’s relatively easy to calculate the size of your current goldfish tank by using the formula below:

  • Measure the length of the tank from left to right.
  • Measure the width of the tank from front to back.
  • Measure the depth of the tank from top to bottom.

Once you have all those measurements, you can use the calculator at this link to work out your fish tank’s size.

The figure you get is the number of gallons that your current goldfish tank holds. Now, use the rules outlined above to work out how many fully grown adult goldfish you can safely keep in your tank.

If the goldfish tank you have turns out to be too small, you’ll need to upsize it to one more suitable.

Final Thoughts

Did you enjoy our guide to goldfish tank size? If you found the article helpful, please take a few moments to share it.

It’s essential that you keep your goldfish in a tank that’s the correct size for them. Goldfish of all types can grow to be quite a large size. 

If the tank is too small, the fish can fail to grow properly or even become deformed. The water quality in a small tank can suffer, too, leaving your fish at risk of nitrate poisoning.

How many goldfish do you keep? What size tank do you have? Tell us about your pets in the comments box below!

3 thoughts on “Goldfish Tank Size Guide”

  1. iS A 40 gallon with 3 fish ok? i will be doing 30 precent water changes each week, and with a polar aurora canister filter (370 gph)

    • 40 gallons is okay for a time, but for 3 adult common goldfish, it may be too crowded, and wouldn’t really be fair to cramp them in for their whole lives.
      It is usually agreed amongst aquarists that a tank size of 50 gallons for 1 adult goldfish, and then an additional 15 gallons for every additional goldfish, so an 80 gallon tank would probably be the ideal size.

      If you go for a smaller breed of goldfish like the pearlscale, you will definitely be able to fit 3 in a 40 gallon aquarium, just be aware that as they are a fancy breed, they can come with health problems and need specific care, pearlscales in particular are susceptible to bloat so keep them on soft, water soaked pellets if you decide to keep them.

      hope this helps.

  2. I have four fancy goldfish in a 20-gallon tank. I have two twenty galIon filters in my tank. I do a water change twice a day after feeding my Goldfish. I also vacume the tank daily. They are all healthy and active, and playful. They even breed. Will they be okay in the 20-gallon tank, or do I need to size up?
    Thank you!


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