Goldfish Tank Size – Everything You Need To Know

If you’re a goldfish owner, then you know that keeping your fish happy and healthy is important. One of the most important things you need to do is make sure your tank is the right size. In this blog post, we’ll tell you everything you need to know about goldfish tank size. We’ll cover what sizes are best, how many fish to keep in each size tank, and more! Keep reading to learn more.

Key Takeaways

  • Goldfish grow too large sizes and need aquariums of at least 20 gallons with 40 gallons being preferred
  • Slim-bodied goldfish grow longer than fancy varieties
  • All goldfish are hard on a bioload due to their mass

Introduction to Goldfish

Chances are, you’ve owned a goldfish before. Whether you won one at a local fair or took care of a pet goldfish as a child, these fish have made their way into the homes of many. The problem is that they’re not exactly fish suitable for the home environment, though.

Goldfish are ancient fish. They have domesticated a millennium ago in China and spread worldwide by the early 1800s. Goldfish are the byproduct of mutations from wild carp native to East Asia, more specifically the crucian carp (Carassius carassius).

It is easy to see how bright orange goldfish were derived from their more musky-colored relatives. Crucian carp can range greatly in color, though they always appear in more natural tones. They are commonly found in hues of brown, green, and yellow.

Every now and then, these fish would exhibit exceptional colors, more like the ones we see on typical goldfish today. Practitioners of Buddhism saved and preserved these fish. Over hundreds of years, mutated crucian carp were collected and bred to show the best yellow and orange colors. By the 1600s, even more, desirable traits, like long fins, took shape as more and more enthusiasts moved their goldfish inside from their outdoor ponds.

Today, there are many breeds of goldfish, all considered members of the Carassius auratus species. This species is usually categorized into two groups: single-tail and double-tail/fancy goldfish.

Single Tail (AKA Slim Bodied)

Single-tail goldfish are some of the most common goldfish to see. These are the typical take-home-from-fair prize fish. Single-tail goldfish are more challenging in some ways than their fancier counterparts due to their immense size and activity. Because of this, they are best kept in outdoor ponds.

Pond Goldfish

Single tail goldfish can grow to be over a foot in length and live up to 40 years. These slim-bodied goldfish are also extremely active and can be seen swimming from one side of the tank or pond to the other.

Popular breeds of single-tail goldfish include:

Double Tail/Fancy

Double-tail goldfish are much more compact and ornate, but don’t be fooled! These round fish are just as messy as single-tail breeds and live almost as long.

Fancy goldfish usually stay under 10 inches in size. They aren’t as active as single-tail varieties as each breed features some body modification. Whether it be longer fins, eye enhancements, or emphasis on other body parts, fancy goldfish are limited in activity; especially ornate fish might even have difficulty swimming correctly. This makes them ideal for indoor aquariums as opposed to pond setups.

Some of the most common fancy goldfish include:

They Need Room To Swim

Although goldfish have been domesticated for over 1,000 years, they haven’t lost their spirit. These fish are incredibly active swimmers, rarely standing still. The only reasons they might slow down in activity are due to illness, old age, or hibernation. Look down into a pond during the summer versus the winter and you’re sure to see a difference (video source).

Not only are goldfish incredibly active fish, but they’re also some of the largest and messiest freshwater fish you can get. Remember, common goldfish can well surpass a foot in length. Contrary to popular belief, these fish won’t stop growing depending on the tank size they’re kept in.

As we’ll see, all these reasons make the idea of keeping goldfish in a goldfish bowl completely absurd.

Recommended Tank Size

The recommended goldfish tank size depends on the type of goldfish you’re keeping. Single-tail breeds that have large and long bodies need a minimum tank size of 40 gallons per fish. Double-tail breeds that have short and compact bodies need a minimum tank size of 20 gallons per fish.

In many cases, these are only beginning tank sizes. Many single-tail breeds will outgrow this size tank in a few years and will do best in a pond setting long-term. Double tail breeds are generally successful in a 20-gallon tank for the duration of their lives as long as the tank is never overstocked and regularly cleaned.

What is the Ideal Tank?

Believe it or not, goldfish tanks do best with a bare minimum tank setup. The most important aspect of a goldfish tank is filtration.

In terms of the actual setup and goldfish care, less if more. These fish thrive in a bare-bottom tank with plenty of swimming space. This means no plants or decorations. Trust us, your goldfish would eliminate most plants you decided to put in anyway!

Besides the filtration, additional tank equipment is usually minimal.

Water Temperature

Goldfish are unique. They are coldwater fish that does best when aquarium water temperatures are between 68 to 74˚F. For most, this means that ambient room temperature will keep the aquarium at the correct temperature.

The bigger concern is maintaining that temperature, though.

Although extremely cold-hardy, goldfish are still susceptible to rapidly changing water temperatures. For this reason, hobbyists may use an aquarium heater to keep the water temperature steady. In contrast, overly hot climates might require the usage of a chiller to keep water temperatures down.

If keeping goldfish in a pond, then there is usually little need to worry about temperature. A chiller may still be needed, though usually, the construction of the pond allows for shaded areas and deeper sections for cool pockets of relief. During the winter, goldfish will safely hibernate at the bottom of the pond no matter the temperature of the air outside.

Goldfish Filtration

Probably the most important aspect of owning goldfish: the filtration. While you might be able to get by with minimal filtration on tropical community tanks with plenty of live plants, goldfish need heavy equipment.

Not only are goldfish messy eaters that create a lot of waste, but beneficial bacteria also have few places to populate due to the absence of plants, substrate, and decorations.

It is strongly recommended to use a canister filter for goldfish tanks to allow for more space for biological, chemical, and mechanical filtration; a hang-on-the-back filter may work, but might be oversized in comparison to the tank.

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Ideally, goldfish filters should be rated for 10x the hourly turnover or close to that; this means that a 20-gallon tank size would need a filter rated for 200 gallons per hour, and a 40-gallon tank size would need a filter rated for 400 gallons per hour.

In general, canister filters are more efficient than hang-on-the-back ones and only need about a 7-8x turnover, though it’s always best to aim for the full 10x.

Water Parameters

Goldfish need good water quality. They are very likely to succumb to ammonia and nitrite poisoning and need frequent tank maintenance. Ammonia and nitrite should always be 0 ppm. Nitrate should be minimal as live plants can’t be used for export.

Otherwise, goldfish are very adaptive fish. They prefer neutral 7.0 pH conditions, but can be kept in slightly hard or soft water. As discussed before, they need a water temperature between 68 to 74˚F, though they can withstand lower temperatures too as long as values don’t fluctuate.

Tank Maintenance

Believe it or not, goldfish are pretty demanding fish. Even though they’re easy to keep, they need good aquarium husbandry.

Goldfish Mouth

How much maintenance you need to perform on your goldfish tank largely depends on how many goldfish are in the aquarium and how efficient the filtration is. In general, a weekly water change of about 20-30% is the bare minimum. Some hobbyists perform several smaller water changes of about 10-15% throughout the week instead of doing a large amount all at once.

It is crucial to keep a goldfish tank clean as waste accumulates. The bottom of the aquarium should be vacuumed to remove fish waste and leftover food. The filtration media should also be rinsed out every couple of weeks.

If you start to notice that the tank smells, your fish becomes uninterested in swimming and eating, or red spots become apparent around your fish’s gills, test for ammonia and nitrite immediately. Poor water quality and tank maintenance will lead to excess amounts of ammonia and nitrite which can quickly become deadly in a goldfish tank.


Goldfish are omnivores. This means that they need a balance of meaty and plant-based foods to give them a happy and healthy life.

Most aquarium companies make goldfish-specific flake and pellet foods. While these will provide your fish with the necessary nutrients to survive, they won’t give your fish different trace elements or the excitement of trying new foods.

The best goldfish diet will have a flake or pellet staple food alongside a variety of live, frozen, and freeze-dried options as well as a selection of fresh vegetables. Goldfish will happily accept various worms and insects along with blanched lettuce and cucumber. Just keep in mind that these foods can be messier than simple flake or pellet foods, so leftovers need to be removed.

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If you want to make your goldfish tank more natural, then you can include live plants. But wait. Didn’t we say that you can’t keep plants in your goldfish aquarium? That’s true, as long as you don’t mind your goldfish eating them.

Live plants are a great source of food for goldfish. Some species of plant, like anacharis (Elodea spp.), have a fast enough growth rate that they can outcompete your goldfish’s appetite. Some hobbyists even set up separate tanks entirely to grow out fresh greens for their fish.

Tank Mates

The best tank mates for goldfish are other goldfish. There are a few reasons why additional fish species end up being incompatible.

1) Temperature. The biggest problem with finding goldfish tank mates is temperature. These fish are some of the most cold-tolerant in the hobby and not much other fish can compare or adapt.

2) Space. Goldfish need plenty of open swimming space and water volume to dilute their waste. Many breeds of double-tail goldfish are also limited in their maneuverability, which other fish might take advantage of, especially during feeding times.

3) Tank setup. All in all, goldfish have a pretty specific tank setup. Many of the tropical fish commonly kept in the hobby require substrate and decorations to feel safe.

4) Aggression. Goldfish are relatively peaceful fish. However, many hobbyists think they can throw a bottom-dweller into their tank with no repercussions. Sadly, there are many horror stories of pleco species sucking the slime coats off goldfish, which can lead to disease and eventual death.

This isn’t to say that hobbyists haven’t successfully kept additional fish in a goldfish tank. Many goldfish keepers have luck with dojo loaches (Misgurnus anguillicaudatus) and white cloud mountain minnows (Tanichthys albonubes). With more fish also comes the need for a larger aquarium.


What size tank do you need for 2?

This depends on the breeds of goldfish you are keeping. If you have two single-tail goldfish, like comet goldfish, then you will need an 80-gallon tank size. If you have two fancy goldfish, like telescope eye goldfish, then you will need a 40-gallon tank size.

What tank size do you need for 3?

Using the same ratio of 40 gallons per single-tail goldfish and 20 gallons per double-tail goldfish (fancy), 3 goldfish would need 120 gallons or 60 gallons respectively.

Is 1 gallon enough?

No! 1 gallon is never enough room for a goldfish due to their size and bioload. Unfortunately, companies target unknowing hobbyists with images and displays of goldfish happily living in small containers. This is the opposite of the truth and all goldfish require an appropriately sized tank to live.

How many can be in a 10-gallon tank?

Again, none. The smallest fancy goldfish varieties require at least 20 gallons on their own. Smaller goldfish can be grown out in a 10-gallon tank size by experienced hobbyists, but an upgrade will be needed within a couple of months.

Final Thoughts

Goldfish tanks are simple and elegant. But the truth is that these can be quite demanding fish and beginners might struggle to meet all their needs.

Most important is a good goldfish tank size. Single-tail breeds need 40 gallons each while double-tail breeds need 20 gallons each. After that, filtration and maintenance will keep your goldfish tank clean and clear of any excess nutrients. Lastly, a well-varied omnivorous diet will keep your fish shining like gold.

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