How Many Goldfish in a 20 Gallon Tank?

Many people don’t realize that goldfish can grow to large sizes, and as a result, their aquariums become overcrowded. As a result, many goldfish find themselves in the unfortunate position of being kept in tiny bowls or tanks that are too small for their long-term health. In fact, goldfish need at least a 20 gallon aquarium to be healthy and thrive.

In this article, we will discuss why 20 gallons is usually the minimum tank size for these popular aquarium fish, how many goldfish can be kept in a tank of this size, and what influences this number.

Goldfish require at least 20 gallons

Goldfish are often considered low-maintenance novice fish that can be kept in small tanks or aquariums. However, this couldn’t be further from the truth. Goldfish are somewhat messy fish that produce a lot of waste and require a lot of care and attention.

One of the most important things to remember about goldfish is that they can grow quite large. Depending on the variety of goldfish, they can reach lengths from 6 inches to 2 feet. As a result, they need a lot of space to swim and move around.

For these reasons, a 20 gallon aquarium is usually the minimum size you should purchase for goldfish. However, it is important to note that the 20 gallon tank size is only a minimum. If you want your goldfish to really thrive, you should consider getting a larger aquarium.

How many goldfish are in a 20 gallon tank?

So, how many goldfish can you keep in a 20 gallon aquarium? We’ll look at the factors that affect this number later, but you can usually keep up to 2 fish in a 20 gallon tank.

This number is based on the idea that you should allocate a gallon of water for every inch of your fish. Common goldfish can grow up to 10 inches in length, so 2 fish is the maximum you can keep in a 20 gallon tank.

Of course, this number is just a general guideline. By maximizing the number of fish in an aquarium, you are putting a lot of stress on the fish and the aquarium itself. As a result, it’s always better to play it safe and give your active fish some extra swimming space.

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What affects the number of goldfish that can be kept in a 20 gallon aquarium?

Now that we’ve answered how many goldfish can be kept in a 20 gallon aquarium, it’s time to discuss some of the factors that can affect this number.

Types of goldfish

The first factor to consider is the type of goldfish you have. As we mentioned earlier, goldfish come in all shapes and sizes. As a result, some varieties of goldfish require more space than others.

For example, comet fish can grow up to 12 inches long while the fantail goldfish peaks at about 8 inches. This means you can comfortably keep 2 fantails in a 20 gallon tank, but you can only keep 1 comet.

Theoretically, more than 2 small breed goldfish can be kept in a 20 gallon aquarium. However, it is important to remember that goldfish are active fish that need a lot of room to swim. As a result, we still recommend sticking to 2 fish as the maximum number for a 20 gallon tank.

Tank Maintenance Frequency

Aquarium cleaning.  Pumping out water from an aquarium.  Close-up.  Siphon gravel cleaner in the aquarium.  The concept of maintenance and hygiene of aquarium fish.

A key part of keeping your fish happy is keeping their aquarium clean. This means regular water changes and gravel vacuuming to remove debris and waste. This contributes to the cleanliness of your fish ecosystem, which contributes to the health of your fish.

The frequency of aquarium maintenance can affect the number of goldfish you can keep in a 20 gallon tank. While goldfish can be extremely hardy, they are still sensitive to their environment. A dirty aquarium can lead to a number of health problems for your fish, including illness and infection.

As fish owners it is our responsibility to create a clean and safe environment for our fish. If you want to keep more fish in your 20 gallon tank, you need to be prepared for more frequent water changes and cleaning.

Tank layout

Another factor influencing the number of goldfish that can be kept in a 20 gallon tank is the layout of the tank. Of the many types of goldfish tanks, two are the most common: planted tanks and unplanted tanks.

Planted tanks usually have more vegetation and decor, which gives the fish hiding places and places to explore. These tanks tend to be more aesthetically pleasing, but they can also mean giving up valuable space in an already crowded tank.

DIY aquarium decoration

On the other hand, unplanted tanks have a more simplified design. This can give your larger fish more room to swim, making the fish feel exposed and stressed.

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The layout of your aquarium can have a significant impact on the number of goldfish you can keep. You may want to stick with a solitary tank if you have a planted tank. And if you have an aquarium without plants, you can keep 2-3 goldfish.

Special Considerations for Small Tanks

Small tanks like 20 gallons come with their own set of problems. In addition to the tips and rules that apply to large aquariums, there are a few other things you need to keep in mind when you set up a small aquarium.

Beware of aggression

Although goldfish are not naturally aggressive, they can become aggressive if they feel stressed or territorial. This is most likely to happen in a small goldfish tank where the fish don’t have enough room to swim and explore. Lack of space can cause your fish to become aggressive towards each other, leading to fights and injury.

To avoid aggression, it is important to give the fish enough room to swim. In a 20 gallon aquarium, we recommend keeping one goldfish or at most two small goldfish. If they are still underage, you can keep 1 to 4 fancy goldfish together, but it’s best to play it safe and keep them separate.

Check your water parameters often

Another thing to keep in mind when stocking a small aquarium is that water quality can change quickly. This is because smaller tanks have less water volume, which means that waste and debris can build up more quickly. As a result, it is important to check water parameters frequently and eliminate potential problems with goldfish in the bud before they become big problems.

First, avoid an overcrowded goldfish tank by sticking to our rule of thumb: 1-2 goldfish per 20 gallons. Then you can start using more creative strategies to keep your aquarium clean. For example, when choosing protein-based fish food, look for products that contain fewer fillers and more natural ingredients. These foods are healthier, but they can also help reduce aquarium waste.

You may also want to consider using a gravel vacuum during water changes. Gravel vacuums can help remove debris and waste from your gravel, which can greatly improve your water quality.

In addition, we recommend using a filter designed for an aquarium that is twice the size of your 20 gallon aquarium. A larger filter will help keep the water clean, but will also help reduce stress on your pet fish.

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Growth may be retarded

In all species, the typical body length of an adult fish in the wild tends to be slightly longer than the average home aquarium can accommodate. In fact, most home aquariums are large enough to accommodate juvenile or juvenile fish. As a result, your farmed goldfish may never reach their full potential if kept in a small aquarium.

ranch goldfish fry
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While this doesn’t necessarily mean your fish will be unhealthy, it’s something to keep in mind if you plan on keeping goldfish in a 20 gallon tank. Hobbyists looking to maximize the growth of goldfish often opt for large tanks, such as 30 or 40 gallons.

But if you decide to keep your goldfish in a smaller tank, don’t worry! There are many more ways to provide them with a high quality of life. By following the tips in this article and carefully monitoring your water parameters, you will be able to provide your pets with the best possible care. After all, aquarium goldfish have a lifespan of 5-10 years, so you will have plenty of time to please your little fish friends.


To sum it up, we recommend sticking with one goldfish or two small goldfish in a 20 gallon aquarium. If you plan to keep fancy goldfish, you can keep 1-4 individuals together if they are still underage. But remember, it’s always better to play it safe and keep them separate.

Also, keep in mind that water quality problems can develop faster in smaller tanks than in larger ones. To keep your fish healthy, we recommend checking the water parameters frequently and using a filter designed for an aquarium that is twice the size of your 20 gallon tank.

We hope this article was helpful to you! Remember, if you have any questions about caring for your goldfish, our expert team is here to help. And to learn more about caring for goldfish, be sure to check out our other articles.

Happy fish keeping!

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