Goldfish Tank Size—How Many Goldfish Per Gallon

By Eddie Waithaka @aquariawise

Updated: 2nd, August, 2022

What is a good size tank for goldfish?

This is a common question that often comes up in both new and experienced fish keepers circles. And one statement you will hear quite often is goldfish need a big tank.

Well, it is true that goldfish require a big aquarium, though you can also keep them in a reasonably small fish tank albeit more tasking.

Some new aquarium owners even maintain them in bowls and nano tanks, which I should say, is ill-advised.

The recommended goldfish tank size generally depends on the species you have.

A goldfish tank should be 3-Feet long with an overall volume of 20-gallons (or more) for one goldfish. A common goldfish may require a longer tank (4-Feet long tank) that is at least 40-gallons for one fish because they often grow bigger than their fancy cousins. If you plan on keeping more than one fancy goldfish, add an extra 12-gallons for each additional fish (between 0.5 to 1.5 inches of goldfish per gallon), and 15-gallons for every common goldfish you add (between 40 and 50 gallons for a one to three goldfish).

That said, let’s delve a little more into goldfish tanks. We’ll look at how to determine the size of a goldfish tank, what this fish like in an aquarium, and appropriate goldfish tank conditions.

But first,

What is a Good Tank (+Size) for A Goldfish

As I’ve mentioned before, goldfish tanks sizes will depend on the goldfish species you want to keep, though your decision will also be influenced by the number of fish you want and the size of your tank, assuming you already have one.

The best goldfish tank is 20 gallons for one fish and up to 50 gallons for a pair or three fish. The tank should have a powerful filter and plenty of plants, but a heater is not a must as long as the temperature remains between 60 and 72 degrees Farenheit. A pH of 7, kH of 70 to 140 ppm and a gH of 50 to 120 ppm is advisable.

Nonetheless, poor accommodation and inadequate care, regardless of your constraints, will contribute to your fish failing to reach their full potential.

The first thing to know is that adult goldfish easily grow up to 12 inches in length. Thus even small babies like the ones available in most pet stores will eventually max out a small aquarium.

Therefore, try to avoid maintaining goldfish in bowls from the start and instead go with a fish tank that is more than 20-gallons. But of course, you’ll need a little more for common goldfish and once your fancy goldfish start to increase in size.

Moreover, it’s worth noting that common goldfish are often more suited to a pond once they grow larger to ensure they get enough room to swim.

Goldfish Tank Size—How Big Should Your Goldfish Tank Be

The recommended 20-gallon tanks for fancy goldfish and 30-gallon for a single common goldfish is a good estimate, but only to the extent, your fish won’t reach maximum length inside this tank.

In which case, if you intend to maintain a goldfish in the starting tank all its life, a good hack is to use the six times the length rule. Where you multiply the maximum potential size of the fish by six (6).

This will ensure your fish has adequate swimming room and water volume all through.

A common mistake by hobbyist is to multiply the baby size, at which you buy a goldfish, by is which is wrong and inaccurate.

Thats said, the estimated full grown adult size of fancy goldfish can be up to a foot long but they commonly stay within the 6 to 8 inches range. Whereas common goldfish reach between 7 to 10 inches and 12 inches when kept in a spacious tank or pond.

However, your goldfish will get to maximum length over a lifespan ranging between 15 to 20 years. Therefore, it’s fine if you start your goldfish in a small fish tank and relocate them as they grow bigger.

Maybe even consider placing them in an appropriate pond once they get to their maximum length.

Fancy Goldfish Tank Size

A fancy goldfish tank should be sized anywhere from 20 gallons for one fish and 70 gallons for a group of four (lightly stocked) or five (heavily stocked). While one will fit a single goldfish, a 40 gallon tank is more appropriate and recommended since goldfish are social and prefer been in a group. Fancy goldfish can grow up to 12 inches in length which further justify the 40 to 50 size aquarium.

You’ll want a tank that can fit the fully length of an adult goldfish and accomodate their flowy tails without squeezing or knocking on items or aquarium walls while swimming.

Common Goldfish Tank Size

The best tank size for common goldfish is 70 gallons because a single can reach 15 inches in length and the fish are social and prefer to be kept in groups. 40 gallons is the minimum size for a single common goldfish but rarely is this enough for most keeper who want more than one goldie.

The same as fany goldfish, you want an aquarium size that will accomodate your common goldfish full length and its fins and leave enough space for accessories, such as filters, plant, heaters, and decorations.

How Many Goldfish Per Gallon of Water

You should have about 0.5 to 1 inch of goldfish per gallon of water in your fish tank, but 1.5 is allowable in a heavily stocked tank. Goldfish not only grow big but they also produce more waste than most aquarium fish hence they need for a substatially big tank. 0.5 to a goldfish inch per gallons translate to about 40 gallons for 2 fancy goldfish and 50 to 60 gallons for common goldfish, with plants, accessories, decorations and tankmates considered.

Remember the minimum tank size for fancy goldfish is 20 gallons and 40 gallons for common goldfish, but because you should keep them more than one, 40 to 70 gallons is more recommended depending on the shape your tank’s configuration.

How Many Goldfish Can You Have in a Tank (How Gallons for Goldfish)

For a start, you should never keep goldfish in a bowl because apart from the fish growing quite big, they also put off a lot of waste, more than normal fish.

Moreso, even if you successfully fit any in a small 10-gallon tank, you will most likely be able to only keep one fancy goldfish in there.

Hence, your barest compromise is to keep one or two fancy goldfish in a 20-gallon aquarium. Or maintain one or two juvenile common goldfish in 40-gallon or more.

Moreover, you will need to keep your aquarium thoroughly filtered and perform regular water changes, sometimes more than twice in a week.

Remember to add 10 gallons of water in your fish tank for every additional fancy goldfish and 15 to 20 gallons for every common goldfish you add in the tank.

Lastly, when calculating your goldfish tank, you need to consider space for decorations, and tankmates in case you want to maintain them in a community.

Plus understand that most small tanks for goldfish are sold as starter kits and are only suitable for a few weeks or months at most. Should you maintain your fish in such a tank for a long time, you will shorten their lifespan to about four years, and the fish will be mostly stunted and malnourished.

Below are a few stocking hacks depending on the size of the tank you have.

1. A 10-gallon aquarium is a fine starter size tank for 1 or 2 baby goldfish, but the fish won’t reach proper adult size unless you place them in a larger aquarium as they grow.
2. In a 20-gallon fish tank, you can maintain a single fancy goldfish, but should you need to add another goldfish, remember to increase your fish tank capacity by 10 t0 15 gallons.
3. In case you want to add other fish species in a goldfish tank, the rule of thumb is 1 gallon of water per inch of fish. Howbeit, this rule assumes all fish have the same bioload and aggression which is not correct. For this reason, you may want to raise the amount to 2 gallons per inch of fish especially when keeping goldfish with other messy species.
4. Since each goldfish generally require an average of 20-gallons of water, 2 to 3 healthy fancy goldfish can live happily in a 40-gallon aquarium, but 50 to 60 gallons are recommended for 2-common goldfish or up to 4 small goldfish.
5. Two full grown fancy goldfish can live in a 40-gallon fish tank, whereas just one common goldfish will fit in this size aquarium.
6. Should you decide to consider stocking by the number of fish as opposed to the tank size, 2 to 3 fancy goldfish should have anywhere from 30 to 40 even 50 gallons of tanks space, whereas the same number of common goldfish will require more than 50 gallons.

Below is a quick table for stocking goldfish according to tank size, goldfish type, and the number of gallons (and liters) in your fish tank (aquarium).

Goldfish Minimum Tank Size

The minimum tank size for a fancy goldfish is 20 gallons and 40 gallons for a common goldfish. However, because goldfish are rarely kept singly, you will need at least 40 gallons for your fancy goldfish and 50 gallons for common goldfish for a pair and increase that amount by 15 to 20 gallons for every extra goldfish you place in your fish tank.

You will also need to rise the minimum gallons of water if you decide to keep your goldfish with other tankmates of a different species by the margin the other fish require.

For Instance, if you decide to keep your goldfish with zebra danios or white cloud minnow, you will need about 15 extra gallons (above the minimum) for a school of 5 to 7 fishes.

What Do Goldfish Like in Their Tanks

Once you determined what size tank to use for your goldfish, it is important to shift your focus to other items you’ll probably need in the tank. Because such items as plants, decoration, and driftwood also need space and risk limiting your goldfish swimming area.

Since goldfish are mid-level dwellers, go for small decorations which include short foreground plants that won’t eat up valuable swimming area.

Aquarium decorations are necessary to keep your fish entertained, make the tank look more put together and provide hiding and spawning areas for your goldfish.

However, goldfish are notoriously infamous for nibbling live plants, therefore, add live aquatic plants that are compatible with the fish. Alternatively, add fake plants instead of living ones.

Soft fake plants instead of plastic plants are also more preferable because goldfish have delicate fins that easily get ripped.

On the other hand, wood is perfect if you want to add a natural look in the tank, while rocks, small plants, and a colorful substrate are good to spruce the bottom and lower-mid levels of the aquarium.

By and large, to decorate your goldfish aquarium best and still have a size big enough for your fish to swim, concentrate your add-ons in the bottom and lower-mid-levels.

Then create a backdrop at the back to conceal unsightly tubes, filters, pumps and other fixture hanged at the back and sometimes the base of the fish tank.

What are The Proper Fish Tank Conditions for Goldfish

Knowing how your goldfish prefer to live is key to maintaining it successfully over and above having an appropriate size aquarium.

For starters, goldfish are cold water fish, meaning they do not require a heater. Consequently, you’ll have more space for your fish if when you are aware of this fact.

The fish prefer their water between 60°F and 72°F and a ph from 7.0 to 7.4.

However, goldfish still requires a filter, and an air pump is also quite necessary. The filter especially needs to have a reliable biological media because of the more than normal ammonia load.

But remember each filter is specific for tank size, and also the cartridges which need to be replaced once a month.

Daily weekly 10 percent water changes for new tanks and 25 percent water changes every two weeks to a month for established tanks are recommended.

Lastly, generally cleaning your fish tank and vacuuming your aquarium gravel will help keep the high ammonia levels down. This way, you can get away with keeping goldfish in a small tank.

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Hope you enjoy keeping goldfish.

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