How Many Fish Can Be In A 20 Gallon Tank: Size-Specific Stocking Guidelines

When stocking a 20-gallon fish tank, it’s important to remember that there are limitations on how many fish can be in the tank. 

The size of the fish, how much filtration is provided by the tank, and how often the tank is cleaned all play a role in how many fish can be in the tank.

In this article, we’ll be discussing how many fish can be in a 20-gallon tank. We’ll also give some stocking guidelines as well as the best possible combinations for a 20-gallon tank.

Finally, we’ll tell you about the best fish for a 20-gallon and how to care for them.

How Many Fish Can Be In A 20-gallon Tank?

A 20-gallon fish tank is a great size for beginners. It’s not too big and not too small. Fish size, filtration, and tank maintenance all are crucial considerations in how many fish can be in the tank.

With that being said, a general rule of thumb is that you can put one inch of fish per gallon of water. So, for a 20-gallon tank, you could put 20 inches of the total length of fish in the tank. However, that’s not always the best way to stock a 20-gallon tank.

Number Of Fish In A 20 Gallon Aquarium

Because it doesn’t count the shape & type of the fish. For example, Maintaining slim-bodied fish like Zebra Danios or Neon Tetras isn’t the same as keeping big Goldfish or Cichlids, although the total length of fish is equal to 20 inches.

Some fish create more garbage than others; hence bio-loads are different. In general, larger fish produce more waste; they need more water. Therefore, when stocking a 20-gallon tank, it’s best to follow some size-specific stocking guidelines.

You can follow the one-inch rule for stocking 1-inch to 3-inch size fish, but beyond 3 inches situation changes a lot.

Generally speaking, you can stock twenty 1-inch fish (matured size) and ten 2-inch fish in your 20-gallon tank. However, you shouldn’t follow such a linear calculation while keeping fish more than 3-4 inches in size. 

Based on other factors, you may be able to stock more or less than the ideal amount in your aquarium. Let’s find out the factors to consider in the following sections.

Factors That Influence How Many Fish You Can Keep In A 20-Gallon Tank

You must consider several factors before stocking your 20-gallon tank with fish. These include:

Adult Size Of The Fish

Adult Size of the fish And A 20 Gallon Aquarium Stocking

The size of the fish is another typical mistake that many people make. When making a purchase, they factor in the fish’s size.

However, don’t take into consideration the fact that a fish will ultimately mature in the aquarium if everything goes according to plan. 

Today’s one-inch Goldfish may measure up to six inches or more in length when fully grown. So, always stock based on the fish’s adult size. 

Oxygenation + Filtration

Filtration And A 20 Gallon Tank Stocking

Another critical factor to consider is how much filtration and oxygenation your 20-gallon tank can provide. A 20-gallon fish tank can be filtered and oxygenated in a number of different ways.

However, the most common way to purify and oxygenate a 20-gallon tank is by using an aquarium filter.

There are a variety of different aquarium filters on the market. But, of course, the type of filter you use will depend on the kind of fish you’re keeping.

For example, if you’re keeping Goldfish, you’ll need a filter that can handle the waste they produce. Canister filters and hang-on-back filters are both excellent choices for goldfish tanks.

On the other hand, if you’re keeping a community tank with smaller fish, you can get away with using a sponge filter or even an internal filter.

The important thing is to make sure your 20-gallon tank is appropriately filtered and oxygenated. A good rule of thumb is to choose a filter rated for a tank that’s twice the size of your 20-gallon tank.

Plants provide additional filtration by reducing nitrates produced from fish poops. Therefore, you could stock a bit more than the ideal amount in a planted aquarium.

The Shape Of The Tank

Tank Shape And A 20 Gallon Tank Stocking

The shape of the 20-gallon tank is another factor to consider when stocking your aquarium. The general rule of thumb is that you can stock one inch of fish per gallon of water.

However, this rule doesn’t consider the fact that some aquariums are wider than they are tall. So, for example, a 20-gallon aquarium that’s wide and shallow will have a different stocking limit than a 20-gallon aquarium that’s tall and narrow.

This is because the surface area of the aquarium plays a role in how much oxygen is dissolved in the water. A wider aquarium will have a greater surface area, which means more oxygen will be dissolved in the water.

On the other hand, a taller aquarium will have a smaller surface area. This means less oxygen will be dissolved in the water. Because of that, some fishkeepers also follow the surface area rule, which is stocking 1-inch fish for every 12 square inches.

It works better for irregularly shaped aquariums like tall or hexagonal-shaped aquariums.

More Read: Bow Front vs. Rectangular Aquarium: Which One to Choose?

Other factors that affect the stocking quantities are:

  1. General Fish compatibility – You must make sure the fish you want to keep are compatible. Some fish are very aggressive, while others are very peaceful.
  2. What Strata of Water Column They Occupy– Different fish occupy different areas of the water column. There are surface-dwelling fish, midwater fish, and bottom-dwelling fish. It’s essential to make sure you have a good mix of fish that occupy different areas of the water column.
  3. How They Interact With the Same Gender of the Species – Some fish are very territorial, like betta fish. Male bettas can’t live together because they fight. Only female bettas can be up in the same tank with caution.
  4. Shyness or Shoaling Behavior – Some fish are very shy and need to be kept in groups. This is especially true for timid fish like neon tetras. Neon tetras should be kept in groups of at least six, but preferably more.
  5. Water Parameters – You should keep fish that have similar tank conditions. You shouldn’t keep tropical fish with cold water fish. For example, goldfish and neon tetras have different temperature requirements, so they shouldn’t be kept in the same tank.

What Are The Coolest (Best) Fish For A 20-gallon Tank

Now that you know how many fish you can keep in a 20-gallon tank, it’s time to choose the coolest fish for a 20-gallon tank. Let’s see what are the viable candidates as 20-gallon tank inhabitants.

When stocking a 20-gallon aquarium, it’s crucial to choose a variety of fish that occupy different areas of the water column and have different swimming behaviors.

Here are some of the best fish for a 20-gallon aquarium:


Tetras in a 20 gallon tank

Tetras are a classic choice for community aquariums. They come in a wide variety of colors and sizes, and they’re relatively peaceful fish. They do best in a group of six or more.


Guppies in a 20 gallon tank

Guppies are one of the most popular aquarium fish for beginners. They’re relatively easy to care for, and they come in various colors and patterns. Guppies are also livebearers, which means they give birth to live young.

It’s possible to keep guppies solely and still make your aquarium appealing because it’s one of the best colorful freshwater fish for a 20-gallon tank.

Celestial Pearl Danio

Celestial Pearl Danio in a 20 gallon tank

Danios are active fish that do best in a group. They come in various colors, but the most colorful is the celestial pearl danio. Danios are relatively easy to care for, and they’re a good choice for beginners.


Platies in a 20 gallon tank

Platies are another popular choice for beginner aquarists. They’re relatively easy to care for, and they come in a wide range of colors. Platies are also livebearers, which means they give birth to live young.

Betta Fish

Betta fish in a 20 gallon tank

The Betta is one of the most eye-catching fish for aquarists because of its vibrant colors, brilliant scales, and ornate fins.

To avoid additional fights, maintain one male species in an aquarium. Betta sorority is a group of female Bettas kept together, however, need to do that carefully. The good news is that bettas can live with calm species but avoid creatures that nibble their fins.

Harlequin Rasbora

Harlequin Rasbora in a 20 gallon tank

The Harlequin Rasbora is a small, peaceful fish well-suited for community aquariums. They’re relatively easy to care for, and they come in a wide range of colors. Rasboras are schooling fish, so they do best in groups of six or more.


Mollies in a 20 gallon tank

Molly is a world-exclusive fish species. Unlike other species, they give birth to their offspring rather than laying eggs.

These beautiful livebearers are calm. The fish are simple to care for and may be kept by a newbie.

Corydoras Catfish

Corydoras catfish in a 20 gallon tank

Corydoras catfish are a classic choice for community aquariums. They’re peaceful, bottom-dwelling fish that do best in groups. In addition, Corydoras catfish are relatively easy to care for. They come in a wide variety of colors and patterns.

What are the Non-Fish Animals You Can Keep In 20 Gallon Fish Tank?

Freshwater Crabs

Freshwater Crabs In A 20 Gallon Aquarium

Freshwater crabs are relatively easy to care for, and they come in a wide range of colors. Crabs are also interesting to watch as they climb and explore their tanks.

Most adults’ carbs are between 1 and 4 inches tall. Most freshwater crabs are omnivores means they’ll consume snails, shrimp, and even small tropical fish. So, while keeping crabs in the tank, keep tank mates relatively bigger in size.


Shrimps In A 20 Gallon Aquarium

Shrimps are one of the most popular invertebrates in the aquarium trade. They come in a wide range of colors and sizes, and they’re relatively easy to care for. Shrimps are also interesting to watch as they swim and explore their tanks.

If you want a shrimp-only tank, you can put 40-60 shrimps of different species. Most shrimp can also coexist with fish if they are big enough to not be eaten.

However, most freshwater invertebrates are susceptible to pollutants in water. So maintain your tank as clean as possible.

Shrimps are a great addition to a tank; not only do they bring colors to the tank, but some species are also great algae eaters, keeping the tank clean.


Axolotl In A 20 Gallon Aquarium

Most axolotls are between 12 and 18 inches long. Therefore a single axolotl requires a 15-20 gallons tank. They are extremely messy. Consequently, you’ll need to boost filtration; however, they are interesting characters.

I suggest mollies as axolotl mates since they are too large for the amphibians to devour and tough enough to endure the axolotl’s muck.

Fish You Shouldn’t Keep In A 20-Gallon Tank


goldfish should not be kept in a 20 gallon tank

Goldfish are large-bodied messy fish, and they thrive in a large aquarium. They grow quickly, and the increased bio-load may be overwhelming for the filtration system in place in a 20-gallon tank. 

So, unless you’re prepared to upgrade to a larger tank or augment the filtration, it’s best to avoid Goldfish.


Angelfish should not be kept in a 20 gallon tank

Due to their immense size and aggressive attitude, angelfish need a tank with a minimum capacity of 20 gallons and an additional 10 gallons for every extra fish. 

They thrive best in a larger tank because a single angelfish can grow up to 12 inches. Therefore adults should never be kept in nano tanks.


oscar fish should not be kept in a 20 gallon tank

Oscars are among the biggest freshwater fish, reaching up to 18 inches in length, making a 20-gallon tank inadequate for them. Oscar’s ideal tank size (minimum) is 50 gallons.

The Oscars don’t feel good in a crowded tank. And their territorial and aggressive nature makes it challenging to co-habit with other fish.

Common Plecos

Common Plecos should not be kept in a 20 gallon tank

Plecos are large, nocturnal fish that can grow up to 15-24 inches. So, while a 20-gallon tank may be suitable for a juvenile pleco, it’s not big enough for an adult.

What Is The biggest fish you can put in a 20-gallon tank?

What big fish can live in a 20-gallon tank? Some fishkeepers like to keep one centerpiece fish for their 20-gallon tank. And they want to know what’s the biggest fish can be put.

A 20-gallon Tank is approximately 24-30 inches long and 12 inches wide, depending on the standard or long tank. So, if you keep a fish that is 6 inches long, then it will only have 18″ by 6″ space for moving around. That’s not a lot of space.

So, ideally, the biggest fish you can keep that grows no bigger than 6 inches long. However, I prefer keeping fish that grow only 3-4 inches maximum.

The Best Possible Combinations For A 20-gallon Tank

When stocking a 20-gallon tank, the best possible combinations are:

  • One to two medium-sized fish
  • Ten to twelve small fish (Schooling fish)
  • Six to eight bottom-feeder fish

Indeed, it’s up to you; before stocking, think about the compatibility and other factors mentioned above to make a wise decision. 

And also, there are some other fish that you can put in your tank besides my list.

Here, I’ll suggest some combinations; you can try them if you like.

Combo-1: Make A Betta Tank

  • 1 Betta Fish (male or female)
  • 4 Corydoras catfish
  • 6-8 Harlequin Rasbora

Combo-2: Livebearer Tank

  • 2 Mollies
  • 3 Guppies
  • 2 Platies

Combo-3: Community Tank With Shrimp

  • 6-8 Celestial Pearl Danios
  • 4-6 Corydoras Catfish
  • 6 Cherry Shrimp

Combo-4: Tetra Tank

  • 5 Neon Tetra
  • 5 Cardinal Tetra
  • 3 Cherry Shrimp
  • 4 Corydoras Catfish

Combo-5: Guppy Tank

  • 6 Guppies (Maintain 1:3 male-to-female ratio)
  • 6 Corydoras Catfish
  • 6 Cherry Shrimp

Best Live Plants For A 20-Gallon Tank

Live plants are great for 20-gallon tanks because they help keep the water quality high and provide hiding places for fish. 

They absorb nitrates and harmful CO2 from the aquarium and help in oxygenation. Some of the best live plants for a 20-gallon tank are:

  • Anacharis
  • Anubias
  • Java Moss
  • Amazon Sword
  • Cabomba
  • Java Fern
  • Hornwort
  • Sword plants

How To Set Up A 20-Gallon Tank

After deciding on what types of fish or combination you’ll keep, you’re ready to set up your 20-gallon tank. The basic features that your tank should have are the following:

  • Enough Hiding places for fish that help to release stress
  • Sound filtration system to keep the water parameters in a healthy range
  • A heater to maintain the consistent tank temperature
  • Substrates – help anchor the plant and house beneficial bacteria.

Step One: Choose The Right Location

The first step is choosing the right location for your 20-gallon tank. It should be away from direct sunlight to avoid an unnecessary rise in tank temperature and algae build-up.

Step Two: Set Up The Tank

Now it’s time to set up the tank. You can buy a 20-gallon aquarium kit for plug & play. Kits usually come with basic filtration, hood, and lighting. 

You can also buy a plain tank and later purchase the gears separately.

Place it on a level surface and add the gravel, rocks, and other decorations.

Step Three: Fill The Tank With Water

Fill the tank with water, and then add the plants (If you wish to have some). Next, add a bacteria starter to begin the cycling process.

Step Four: Put In The Filter, Heater, And Light

You’ll need a filtration system to keep the tank clean. You can read my other article on the best fish tank filter for 20 gallons. The type of aquarium filter you need depends on the fish combination you want to keep.

A heater is an essential gadget for maintaining the tank temperature constant. Know your fish preference and set the temperature accordingly.

If you want to grow plants, you need LED lights to provide the right spectrum for plant growth. In addition, you may need to use fertilizer or root tabs for thriving plant growth.

Step Five: Add The Fish

It’s now time to add your fish. Start with a few fish and then add more later.

And that’s it! These are the steps you need to follow to set up your 20-gallon tank.

Caring For Your 20-Gallon Tank

Once you have set up your 20-gallon tank, here are a few things you need to do to care for your fish:

  • Feed them twice a day, but only give them as much food as they can eat in a few minutes.
  • Vacuum the gravel and clean the filter every week.
  • Every other week, do a 20 percent water change.
  • Keep an eye on the water quality and temperature and make sure they are stable.

Frequently Asked Question

Is a 20-gallon tank big enough for 2 goldfish?

As I already mentioned, Goldfish isn’t an ideal choice for a 20-gallon tank. They are messy and produce a lot of waste. They need a much larger tank to thrive as they grow really big and poop a lot. However, if you still want to keep Goldfish, you can put 2-3 of them, but make sure you provide adequate filtration.

How Many Neon Tetras Can I Put In A 20-gallon Tank?

You can put 15-20 neon tetras in a 20-gallon tank. They are small fish and do not produce much waste.

How Many Fish Are Too Many In A 20-Gallon?

Suppose you are slowly adding fish and want to know whether you have stocked the right amount for your setup. I would suggest performing a water test and looking for ammonia and nitrate. As long as ammonia is zero and nitrate is below 20 mg/L, all are ok.

Wrapping Up

So, how many fish can be in a 20-gallon tank? You can keep around 20 small fish (1-inch adult size) in a 20-gallon tank. But remember, the amount of fish you put in your tank will also depend on their size, temperament, and how much space they need to swim.

If you’re still unsure about what kind of fish to get for your tank or have any questions about maintaining a healthy aquarium, please leave a comment below.

I love getting feedback from readers and helping newbies get into this fantastic hobby.

Sujit Modak

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