How Many Goldfish in a 20 Gallon Tank?

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Many people don’t realize that goldfish can get quite large, and as a result, they overcrowd their tanks. As a result, many goldfish find themselves in the unfortunate position of being kept in tiny little bowls or aquariums that are too small for their long-term health. In reality, goldfish need at least a 20-gallon tank to be healthy and thrive.

In this article, we will discuss why 20 gallons is generally the minimum tank size for these popular aquarium fish, how many goldfish you can keep in a tank of that size, and what affects that number.

Goldfish Require At Least 20 Gallons

Goldfish are often considered “low-maintenance” beginner fish that can be kept in small bowls or aquariums. However, this couldn’t be further from the truth. Goldfish are somewhat messy fish that produce lots 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 fish tank is generally the minimum size tank that you should get for goldfish. However, it’s important to note that the 20-gallon tank size is only the minimum. If you want your goldfish to really thrive, you should consider getting a larger tank.

How Many Goldfish in a 20 Gallon Tank?

So, how many goldfish can you keep in a 20-gallon fish tank? We will get into the factors impacting this number later on, but you can generally 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 fish you have. Common goldfish can grow up to 10 inches in length, so 2 fish would be the maximum number you could keep in a 20-gallon tank.

Of course, this number is just a general guideline. By maxing out the number of fish in your tank, you are putting a lot of stress on your fish and the aquarium itself. As a result, it’s always best to err on the side of caution and give your active fish a little extra room to swim.

What Affects The Number Of Goldfish You Can Keep In A 20 Gallon Tank?

Ranchu goldfish

Now that we’ve answered how many goldfish can you keep in a 20-gallon tank, it’s time to discuss some of the factors that can impact this number.

Goldfish Species

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

For example, comet fish can grow up to 12 inches long, while a fantail goldfish maxes out at about 8 inches. This means that you might be able to keep 2 fantails in a 20-gal tank comfortably, but you would only be able to keep 1 comet.

In theory, you could keep more than 2 goldfish of smaller varieties in a 20-gallon tank. However, it’s important to remember that goldfish are active fish that need a lot of space to swim. As a result, we would still recommend sticking to 2 fish as the maximum number for a 20-gallon tank.

Frequency Of Tank Maintenance

Cleaning the aquarium. Pumping water out of the aquarium. Close-up. Siphon Gravel Cleaner Tool In The Aquarium. Aquarium fish content and hygiene concept.

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

The frequency of tank maintenance can impact the number of goldfish you can keep in a 20-gallon aquarium. Though goldfish may be extremely hardy fish, they are still sensitive to their environment. A dirty tank can lead to several health problems for your fish, including diseases and infections.

As fish owners, it’s 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 to perform more frequent water changes and cleanings. 

Tank Layout

Another factor that can impact the number of goldfish you can keep in a 20-gallon tank is the aquarium’s layout. Of the many types of goldfish tanks, the two most common are planted tanks and unplanted tanks.

Planted tanks typically have more vegetation and decor, providing your fish with 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 overcrowded fish tank.

hand placing aquarium decor

On the other hand, unplanted tanks have a more simplistic design. This can provide your larger fish with more swimming space, making your fish feel exposed and stressed.

The layout of your tank can have a significant impact on the number of goldfish you can keep. You might want to stick to a solo tank if you have a planted tank. And if you have an unplanted tank, you might be able to keep 2-3 goldfish.

Special Considerations For Small Tanks

Small tanks, like 20-gallon tanks, come with their own set of challenges. In addition to the tips and rules that apply to larger tanks, there are a few other things you need to keep in mind when you’re stocking a small tank.

Watch Out For Aggression

Though goldfish aren’t aggressive fish by nature, they can become aggressive if they feel stressed or territorial. This is more likely to happen in a small goldfish aquarium where your fish don’t have adequate space to swim and explore. The lack of space can cause your fish to become aggressive towards each other, leading to fighting and injuries.

To avoid aggression, it’s important to provide your fish with plenty of space to swim. In a 20-gallon tank, we recommend sticking to a single goldfish or at most, two small goldfish. If they’re still juvenile, you may be able to keep between 1-4 fancy goldfish together, but it’s best to err on the side of caution and keep them separate. 

Test Water Parameters Frequently

Another thing to keep in mind when you’re stocking a small tank is that water quality can change quickly. This is because small tanks have less volume of water, which means that waste and debris can build up more quickly. As a result, it’s important to test your water parameters frequently and nip potential goldfish issues in the bud before they become big problems.

Avoid having an overcrowded goldfish tank in the first place by sticking to our 1-2 goldfish per 20 gallons rule. Then, you can start employing more creative strategies to keep your tank clean. For instance, when selecting protein-based fish food, look for foods that contain fewer fillers and more natural ingredients. These foods are healthier, but they can also help reduce the amount of waste in your tank.

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

In addition, we recommend using a filter that’s designed for a tank that’s twice the size of your 20-gallon tank. A bigger filter will help keep your water clean, but it will also help reduce the amount of stress on your pet fish.

Growth May Be Stunted

Across all species, the typical adult fish body length of wild specimen tends to be a little longer than the average home aquarium can accommodate. In fact, most home aquariums are only big enough to accommodate the juvenile or adolescent stage of a fish’s life. As a result, your grown goldfish may never reach their full potential size-wise if kept in a small tank.

ranchu goldfish fry
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While this doesn’t necessarily mean that your fish will be unhealthy, it’s something to keep in mind if you’re considering keeping goldfish in a 20-gallon tank. Hobbyists looking to maximize goldfish growth often opt for larger tanks, like 30-gallon or 40-gallon tanks.

But if you’re set on keeping your goldfish in a smaller aquarium, don’t worry! There are still plenty of ways to provide them with a high quality of life. By following the tips in this article and keeping a close eye on your water parameters, you can give your pets the best care possible. After all, the lifespan of aquarium goldfish is 5-10 years, so you’ve got plenty of time to enjoy your little fish friends.

The Takeaway

To sum it up, we recommend sticking to a single goldfish or two small goldfish in a 20-gallon tank. If you’re considering keeping fancy goldfish, you may be able to keep 1-4 together if they’re still juvenile. But remember, it’s always best to err on the side of caution and keep them separate.

In addition, don’t forget that small tanks can experience water quality issues more quickly than larger tanks. To keep your fish healthy, we recommend testing your water parameters frequently and using a filter that’s designed for a tank that’s twice the size of your 20-gallon tank.

We hope you found this article helpful! Remember, if you have any questions about goldfish care, our team of experts is always here to help. And to learn more about goldfish care, be sure to check out our other articles.

Happy fish keeping!

Wanda is a second-generation aquarist from the sunny tropics of Malaysia. She has been helping her father with his freshwater tanks since she was a toddler, and has fallen in love with the hobby ever since. A perpetual nomad, Wanda does her best to integrate fish-keeping with her lifestyle, and has taken care of fish in three different continents. She loves how it provides a nice break from the hustle and bustle of life.

1 thought on “How Many Goldfish in a 20 Gallon Tank?”

  1. I kept 10 feeder comet goldfish I rescued in the 20 gallon tank. Initially I thought the tank would be too small and eventually I have to get new one, 5 years now and not single one of them die. They are all stunted so they have not grown in size at all. I thought the water would be super filthy but like any other aquarium fish, the water replacement can be done bi-weekly and its fine. The Stunted goldfish appears to have healthy appetites as I feed flakes and vegetables and they gobble up like no tomorrow. Hopefully this experience is not unique and for those who want to keep couple more goldfish together in a smaller tank, they can be encourage to do so.


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