How Many Goldfish Can You Keep in a 20-Gallon Tank?

Alison Page

Alison Page


How Many Goldfish in a 20 Gallon Tank

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When you buy a goldfish from a pet store, you might not realize that the cute little inch-long fish you took home will quickly grow. So, many goldfish end up living in tanks that are far too small, leading to long-term health problems.

Ideally, goldfish need a 20-gallon tank as a minimum requirement. So, how many goldfish can you keep in a 20-gallon aquarium? And what factors affect that number?

Read this guide to find out more!

Goldfish Need a 20-Gallon Tank!

The first thing to understand is that the traditional goldfish bowl is unsuitable for keeping goldfish! Goldfish need at least a 20-gallon aquarium to be comfortable and happy.

How so?

  • Goldfish quickly grow too large for a bowl and need lots of dissolved oxygen in the water. The small surface area offered by a bowl simply doesn’t accommodate that requirement.
  • Depending on the variety you choose, goldfish can grow pretty large. A fantail fancy goldfish can grow to 8 inches in length, and a slim-bodied, Comet or Shubunkin can measure up to 2 feet long!
  • Slim-bodied types are better off in a large garden pond, and fancies need a fish tank of at least 20 gallons.
  • Goldfish are dirty fish, and a small tank means more time-consuming maintenance for you and poorer water quality for the fish. These fish also produce lots of waste. 
  • As a result, they require regular water changes to ensure a safe environment. Fresh water increases their life expectancy. 

How Many Goldfish in a 20-Gallon Tank?

So, how many goldfish can live in a 20-gallon aquarium?

As a general rule, you can keep two goldfish in a 20-gallon fish tank.

The rule of thumb for fishkeeping states that you should allow one gallon of water per inch of fish. However, remember that goldfish can reach 8 inches in length and bigger, depending on the variety you’re keeping.

So a 20-gallon tank is a bare minimum you need for these fish.

Bear in mind that by maxing out the number of goldfish in your setup, you risk stressing out the fish and overloading your filtration system.

So, it’s always best to buy the largest tank you can accommodate in your home or just keep two fancy goldfish types in a 20-gallon tank.

Factors Affecting Goldfish Numbers in a 20-Gallon Tank

So, now you know how many goldfish you can safely keep in a 20-gallon aquarium, let’s look at the factors that can influence that number.

What Goldfish Variety?

The first thing to consider is what variety of goldfish you want to keep.

As previously mentioned, fancy goldfish, such as Black Moors, Ryukins, and Telescope Eyes, can grow to measure from 5 inches to 8 inches in length.

These goldfish types are typically clumsy, slow swimmers that don’t need huge amounts of space to swim in. They can survive well in minimum-size tanks with fewer gallons of water. 

However, fancy goldfish do best in a long, shallow tank to easily reach the surface to feed. That said, you could keep two fancies in a 20-gallon tank, provided you don’t fill the space with decorations and plants.

Slim-bodied goldfish types can grow from 12 inches to 2 feet in length. So, if you want to keep these varieties, unless you have a very large tank, a garden pond is a much better option for them.

Of course, in theory, you could keep one Comet goldfish in a 20-gallon tank, but goldfish are known to be gregarious, sociable creatures that ideally should be kept in groups. In Switzerland, it’s illegal to keep one goldfish alone!

Tank Maintenance

Goldfish are messy creatures that generate a lot of waste. That means you must run an efficient filtration system with an appropriate GPH (Gallons Per Hour) rate to cope with a 20-gallon tank.

Ideally, you want a filter that passes the water through the system and around the tank at least four times every hour. However, a filter system with a higher GPH is recommended in a goldfish tank.

Goldfish are hardy fish, but they are sensitive to water conditions. So, you need to keep on top of tank and filter maintenance.

If you want to keep other fish in your 20-gallon tank, temperate, tropical species, such as White Cloud Mountain Minnows, you’ll need to carry out more frequent water changes.

Lots of water changes ensure healthy fish, which complies with the fish rule from fish experts. Air filters and box filters are crucial in providing an ideal environment that provides a comfortable life for the fish. 

Tank Decoration

When it comes to decorating a fish tank, most hobbyists opt for a planted tank with a few stones, pieces of driftwood, and the like. That type of tank setup looks much more aesthetically pleasing than a bare aquarium that’s bereft of any kind of decoration.

Your fish will also appreciate the novelty of things to explore and plants to nibble on.

However, that means giving up space in the tank, which could be a problem if you already have lots of fish in the tank.

On the other hand, a tank with no decoration might provide more swimming space and room for your fish.

However, it will also make your fish feel exposed and stressed.

So, if you want to keep two goldfish in your 20-gallon tank, you’ll need to keep your decorations to the perimeter so that there’s enough space in the middle of the water column for the fish to swim in.

You can use lots of plants as decorations as well. Your tank size determines the level of decoration you can use.

Test Water Parameters in a 20-Gallon Tank

How Many Goldfish in a 20 Gallon Tank

Something else to keep in mind with a small tank is that the water parameters and quality can change very quickly.

As mentioned earlier, small aquariums have less water volume, so debris, waste, and pollutants tend to accumulate more quickly than in a larger setup.

For that reason, you need to test the water parameters often and ensure that the levels of ammonia and nitrites are zero and nitrates are below 30ppm.

You can help maintain good water conditions for your goldfish by not overfeeding them fish and feeding them high-quality food free from artificial additives and padding. That means less waste that will pollute the water.

You should also take time to vacuum the gravel during your weekly water changes to remove accumulations of fish waste, uneaten food, and general debris.

Poor Growth and Failure To Thrive

Although it’s not strictly the case that fish grow to the size of the tank they’re kept in, it’s true to say that fish kept in a very small tank or overcrowded conditions generally fail to thrive, and their growth is often stunted.

Ideally, you want a 30 or 40-gallon tank if your fancy goldfish are to achieve their maximum size.

As mentioned, slim-bodied goldfish will grow much larger if kept in a spacious outdoor fish pond.

In addition, fish kept in overcrowded conditions are more prone to outbreaks of common fish diseases, such as Ich and fin rot.

Final Thoughts

Did you enjoy our guide to how many goldfish you can keep in a 20-gallon tank? If you found the article helpful, please take a moment to share it.

You can keep two fancy goldfish in a 20-gallon fish tank. Slim-bodied goldfish are better suited to life in a large garden pond because of the size the fish can achieve when fully grown.

That said, if you can accommodate a larger fish tank of 30 to 40 gallons in your home, that’s a better choice for goldfish.

The larger the tank, the more likely your fish will reach their full size and thrive.

What size tank do you have for your goldfish? Tell us about your setup in the comments box below.

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