How Many Goldfish Per Gallon (& How To Calculate It)

When adding goldfish to your aquarium, it’s important to know how many goldfish per gallon can fit. The answer mostly depends on the size of the goldfish and maintaining clean water. But remember, there are other factors.

So, in this article, we’ll talk about all the details of how many goldfish you should keep per gallon!

Key Takeaways:

  • The question of how many goldfish per gallon should be kept in an aquarium depends on factors such as the size of the goldfish, their species, and the cleanliness of the water.
  • Various calculations are suggested for determining the number of goldfish per gallon, including the one gallon per inch of goldfish, 20 gallons per goldfish, and 20 gallons for the first fish plus 10 gallons for each additional fish.
  • For larger types of goldfish like Shubunkins or Comets, the recommended calculation may increase, such as 30 gallons for the first fish and 15 gallons for each additional one.
  • The surface area method, based on a 1948 book called “The Goldfish,” suggests 24 square inches of surface area per inch of fish as an ideal calculation.

So, How Many Goldfish Should I Keep Per Gallon Of Water?

There is a good reason for there being so many recommendations when it comes to gallons for goldfish. It isn’t as important as it would be for something like cichlids. Stories and anecdotes abound from a single goldfish living happily in a 2½ gallon tank to a school of 10 goldfish in a 40-gallon tank.

Of course, the species of goldfish you have and the maximum size they can grow to should be a top priority. While the following calculations are suggestions, you should refer to the specific requirements of your particular goldfish.

Tank Size Guidelines
– 15 gallons: Two 2 to 2½ inch goldfish
– 20 gallons: Two 2½ to 3 inch fish
– 30 gallons: Three 3 inch fish
– 40 gallons: Four 3 inch fish
– 50 gallons: Five 3 inch fish

One Gallon per Inch of Goldfish

A simple calculation that many people seem to abide is by going with one gallon per inch long of goldfish. This allows for various species, sizes, and shapes of goldfish, including ones with hoods. But, this may only be ideal for mature fish, since it doesn’t provide much information for juveniles.

20 Gallons per Goldfish

Another easy calculation is to go with 20 gallons per goldfish across the board. This way, you can anticipate juvenile growth into maturity while maintaining proper water parameters. But, that means you’ll need a 40-gallon tank for two goldfish. That’s quite big for so few fish.

In the case your fish are fancy and large, this may be the most ideal calculation to use. Alternatively, it may be okay for juveniles without filtration or aeration. Once they start growing, aerating will be necessary.

20 Gallons for the First, 10 Gallons Thereafter

There’s yet another calculation that seems to be popular among aquascape hobbyists. This is going with 20 gallons for the first fish and then adding 10 gallons for every fish thereafter. In this case, three fish will fit into a 40-gallon tank.

For Larger Types of Goldfish

For larger types of goldfish species, such as Shubunkins or Comets, the equation must increase because these can reach up to 10 long inches or more. So, 30 gallons for the first fish with 15 gallons for each additional one thereafter should suffice.

Calculations via Surface Area

There is one method people find tried and true. It comes from a 1948 book called, “The Goldfish” by Harvey and Hems. This states that there should be 24 square inches of surface area per inch of fish. This is the most ideal calculation in the event the tank doesn’t have aeration or water flow.

However, it’s important to note that using the surface area rather than the gallon size will support the same number of fish. For instance, let’s say there are two tanks. One 30 gallons and another that’s 50 gallons but both have a surface area of 288 square inches.

Regardless of the gallon size, each will only be able to supply enough oxygen for one goldfish up to 12 inches long. The calculation for this is as follows:

Total Square Feet of Tank Surface Area ÷ 24 Square Inches per Fish = Total Possible Length of One Fish

288 ÷ 24 = 12

how many goldfish per gallon

Quick Reference

If you’re not one for math or lack the space for precise measurement, consider the list below as a quick guideline. These are for tanks that will include a filtration system with average-sized goldfish:

  • 15 gallons – two 2 to 2½ inch goldfish
  • 20 gallons – two 2½ to 3 inch fish
  • 30 gallons – three 3 inch fish
  • 40 gallons – four 3 inch fish
  • 50 gallons – five 3 inch fish

Proportions Are Somewhat Insignificant

The shape of the aquarium and how clean it is are going to be more important than tank size. Since goldfish love to be around others, they’ll be happy slightly cramped as long as the water stays pristine.

Water quality and maintaining the right parameters will be essential. This will be especially true when keeping a school of fish in a smaller tank smaller than what’s advisable. Every type of goldfish will have slightly different needs, but the parameters below are a good, general range:

  • Ammonia: 0 ppm
  • Nitrites: 0 ppm
  • Nitrates: 30 ppm or less
  • pH Balance: 7.0 to 8.0
  • KH: 50 to 120 ppm
  • GH: 100 to 300 ppm

How Many Goldfish Will Be Happy In A 10 Gallon Tank?

The number of goldfish that can fit in a 10-gallon tank will largely depend on the breed, size, and the number of fish. For small juveniles, two to four will fit but they won’t grow to their proper mature size unless they have a bigger tank.

Since most goldfish grow somewhere between six and eight inches, only one adult fish will be comfortable in a 10-gallon tank. But, two will fit comfortably with smaller breeds. If it’s a larger one, like a Comet, a 10-gallon tank will not be suitable.

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How Many Goldfish Will Fit In A 20 Gallon Tank?

The number of goldfish for a 20-gallon tank will have a similar calculation to a 10 gallon one. For the average size of six to eight inches, two to three fish should be comfortable. If the goldfish is the kind that can get as big as 12 inches, then one or two are ideal.

But, if you’re just looking to temporarily house the goldfish starting as juveniles, then eight to 10 goldfish will suffice.

Now, check out this video by Fancy Goldfish Fantics on how many goldfish you should have!

What Are the Various Sizes of Goldfish Breeds?

The best and surest way to ensure the proper aquarium size is by knowing how long a goldfish can grow to. Some breeds are small while others are rather large. The types below provide a basis to start from:

  • Bubble Eye: juveniles are one to two inches and grow between four to six inches
  • Butterfly Tail: juveniles are one to two inches and grow between seven to eight inches
  • Celestial Eye: juveniles are one to two inches and grow between four to six inches
  • Comet: juveniles are two to four inches and grow between 10 to 12 inches
  • Common: juveniles are two to four inches and grow between 10 to 12 inches
  • Fantail: juveniles are one to two inches and grow between six to eight inches
  • Jikin: juveniles are two to four inches and grow between eight to 10 inches
  • Lionhead: juveniles are one to two inches and grow between five to eight inches
  • Nymph: juveniles are one to three inches and grow between 10 to 12 inches
  • Oranda: juveniles are two to four inches and grow between seven to nine inches
  • Pearlscale: juveniles are one to two inches and grow between four to six inches
  • Pompom: juveniles are one to two inches and grow between four to six inches
  • Ranchu: juveniles are one to two inches and grow between five to eight inches
  • Ryukin: juveniles are two to four inches and grow between six to 10 inches
  • Shubunkins: juveniles are two to four inches and grow between 10 to 12 inches
  • Tamasaba: juveniles are one to two inches and grow between eight to 10 inches
  • Telescope: juveniles are two to four inches and grow between seven to nine inches
  • Tosakin: juveniles are one to two inches and grow between four to eight inches
  • Veiltail: juveniles are one to two inches and grow between seven to eight inches
  • Watonai: juveniles are two to four inches and grow between 10 to 12 inches; can get to as long as 19 inches in pond settings

Why Do Goldfish Need a Proper-Sized Tank?

Goldfish need a properly sized tank, which means the right number of goldfish per gallon for several important reasons:

  1. Space for Growth: Goldfish can grow significantly throughout their lives, and a properly-sized tank provides enough space for them to reach their full size. Inadequate space can stunt their growth and lead to health problems.
  2. Oxygen Supply: In a larger tank, there is more water volume, which means a higher capacity for oxygen. Goldfish require well-oxygenated water to thrive, and a properly-sized tank helps maintain optimal oxygen levels.
  3. Waste Dilution: Goldfish produce waste, primarily in the form of ammonia. In a larger tank, the waste gets diluted more effectively, preventing the buildup of harmful substances. In smaller tanks, waste can accumulate quickly, leading to poor water quality.
  4. Social Behavior: Goldfish are social creatures and benefit from the company of other goldfish. A larger tank provides the space for a suitable number of goldfish, allowing them to exhibit natural social behaviors and reducing stress.
  5. Water Parameters Stability: Larger volumes of water are more stable in terms of temperature, pH, and other water parameters. Stability is crucial for the health of goldfish, as fluctuations can stress them and compromise their immune system.
  6. Behavioral Enrichment: Goldfish are active and curious fish. A properly-sized tank allows for the inclusion of decorations, hiding spots, and swimming areas, providing mental stimulation and behavioral enrichment for the fish.
  7. Disease Prevention: Overcrowded conditions in a small tank can contribute to stress, making goldfish more susceptible to diseases. A properly-sized tank with appropriate filtration helps maintain a healthier environment, reducing the risk of illnesses.
  8. Longevity and Well-being: A suitable living environment supports the overall well-being of goldfish, promoting their longevity. It allows them to express natural behaviors, reduces stress, and minimizes the risk of health issues associated with cramped or suboptimal conditions.

FAQ

How Big Does an Aquarium Need to Be for 2 Goldfish?

On average, two goldfish should fit in a 10-gallon tank. But, if they are larger breeds, then somewhere between 20 to 40 gallons will be best.

Recap

There are many ways to go about answering how many goldfish you should keep per gallons, especially if you consider the major factors. However, goldfish are a fairly easy breed in general and professional aquarists give suggestions across the board. But, it really does come down to keeping the tank clean.

However, for a basic run-of-the-mill guideline, it seems that the maximum size a goldfish can grow to will be better on which to make an estimation. Therefore, about two fish per 10 gallons for small breeds and 20 gallons for larger ones.

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About the author

Hey! I'm Antonio!

Betta fish keeper for over 6 years now! Since owning a betta I've also housed all kinds of tropical fish, and have seen all manner of problems and how to look after them!

If you need any advice you can always message me or better yet join the Facebook group where a community can answer your questions!