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10 Best Fish for a 20-Gallon Tank (Plus Bonus Critters)

Eric is an aquarium enthusiast with over two decades of experience caring for a wide array of tropical fish.

Cory catfish are among the best fish for a 20-gallon tank.

Cory catfish are among the best fish for a 20-gallon tank.

Stocking Your Freshwater Aquarium

A 20-gallon aquarium offers some exciting possibilities. A tank this size lets you include live plants and have a wide array of beautiful freshwater fish and interesting critters, all without taking up a huge amount of space. But, like any aquarium, if you want it to succeed, you need to stock it wisely.

Choosing the best fish for your 20-gallon tank is about more than just knowing which fish grow to an appropriate size. It’s also important to consider the temperament and needs of any fish you stock and to make sure they are compatible with each other.

The intent of this article is to help you make those decisions. I’ve outlined some of the top fish I’d consider for a tank this size, as well as some of the potential pitfalls. Research is the most important thing when stocking a fish tank. Every fish, critter, plant or decoration you add to your tank plays a role in the overall health of the aquarium.

The idea is to create a tiny ecosystem that requires minimal help from you to keep it going. There are many reasons aquarium fish die too soon, and proper stocking, along with solid tank maintenance practices, can help you avoid most of them.

This post will get you started.

How Many Fish for a 20-Gallon Tank?

It’s wise to get any notions of a specific number of fish for your tank out of your mind. For one thing, new and intermediate fish keepers often over-estimate. We all get excited about different fish species and want to keep as many as possible, but we have to be smart and realistic if we want our tank to succeed.

Instead, as I mentioned above, take some time to research the fish you intend to stock. Learn about their needs and temperaments, and figure out if they can get along.

In my article on the best fish for a 55-gallon tank, I advised readers to consider the different zones of your tank. Some fish spend most of their time on the bottom, some in the middle, and some at the top of the water.

You can do some of that type of planning with a 20-gallon tank as well. This is especially important if you are trying to choose the right fish for a 20-gallon long tank. These tanks are a bit shallower, and you don’t want to overcrowd your fish.

While researching you'll also want to consider:

  • Temperament: Aggressive, predatory, semi-aggressive or community? It's important to match the appropriate fish together. One predatory fish in a tank full of small community fish will wreck your whole aquarium.
  • pH requirements: While most tropical fish can adjust to different pH levels, each species has a specific range where it thrives.
  • Temperature: Tropical fish do best with temperatures in the high 70s, but there are some cold-water species that prefer cooler temps. Don't try to mix and match.
  • Types of fish: Match tropical to tropical and cold water to cold water. Goldfish should not be mixed with tropical fish. African cichlids should be kept with others of their kind.
  • Size: Big fish eat little fish. Don't mix them up and expect good results.
  • Social behavior: Some fish are loners, some should be kept in pairs or small groups, and some are shoaling fish that need to be kept in larger numbers to reduce stress.

Top 10 Fish for a 20-Gallon Aquarium

The fish listed in this article are good community fish. Most are peaceful, but a few have a semi-aggressive side in certain situations. Remember: Do further research before adding them to your tank, so you can be sure they are compatible with other fish you intend to stock.

Here is my list. You can scroll down to learn more about each fish:

  1. Platy
  2. Swordtail
  3. Molly Fish
  4. Cory Catfish
  5. Dwarf Gourami
  6. Neon Tetra
  7. Zebra Danio
  8. Fancy Guppy
  9. Otoconclus
  10. Harlequin Rasbora
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1. Platy

Platies are active fish and a lot of fun to watch. They come in some attractive varieties, including the Mickey Mouse platy that looks like it has the iconic mouse ears on its tail.

It’s a good idea to have a ratio of at least two females for every one male. You can tell males from females by looking at the fin under their bellies. If it is wide and flat the fish is female, where on males it is pointy.

2. Swordtails

Swordtails are similar to platies in both appearance and behavior, though a bit slimmer and sleeker. Like platies, you’ll want only one male for every two females. This reduces the stress on the females when the males get a little amorous and start pestering them.

Male swords are easy to spot by their sword-like tails. I’ve run into fish store employees who didn’t know this, let alone how to sex platies, so they may require a little coaching when you are selecting your fish.

3. Mollies

Molly fish are available in many varieties, such as the sailfin molly, lyretail molly, marble molly, and (my favorite) the black molly. As with the platies and swords, you want a minimum of two females to each male, and you can tell the difference using the same method as with platies.

All three of these fish are livebearers, meaning they don’t lay eggs. It also means you may wake up to baby fish puttering around your tank one morning!

4. Corydoras

Cory catfish are bottom scavengers, and a nice addition to any tank 10-gallons or bigger. These little guys will help keep your tank clean. They’ll eat leftover fish food that falls to the bottom of the tank, but you should include some sinking pellets to be sure they get enough to eat. They are shoaling fish, and should be kept in groups of at least six. For a 20-gallon tank you might consider a few different varieties.

5. Dwarf Gourami

Dwarf gouramis are the only fish on this list that I would consider semi-aggressive at times. They are beautiful, mostly peaceful and can be excellent community fish. However, if you have a pair they may fight, or one may bully the other.

To reduce the chances of this happening try to avoid having two males in the same tank. It is also a good idea to include some good hiding spots in your aquarium so a bullied fish has a place to hide.

6. Neons and Other Tetras

Most tetras, like neons, are small, peaceful fish that need to be kept with others of their kind. A school of a dozen or more neons in a well-planted tank looks amazing, but be sure to have at least six.

There are other types of tetras you might consider as well. Some are similar in size and behavior to neons, such as black neons and cardinal tetras. Others are a little bigger, like black-skirt tetras.

7. Zebra Danio

Zebrafish are small, hardy little striped fish. They look somewhat like tetras, but they are actually minnows. They are super easy to care for, and so they are among the best tropical fish for beginners. They’re also very active little fish, and a lot of fun to have in a tank.

As with tetras, they are shoaling fish who should be kept in groups of at least six. This social environment helps reduce stress and keep fish healthy when they feel threatened.

8. Fancy Guppies

Fancy guppies are some of the prettiest and brightest fish you can stock in your 20-gallon tank. Since they are livebearers you'll shoot for two females for each male. With livebearers, the total number of each fish you strive to keep should always be divisible by three.

These guys are going to be all over your tank. No worries, though. They’re too small to get into too much trouble. While they are extremely active they are very peaceful, and most other small community fish make excellent tank mates for guppies.

9. Otocinclus

Otos are cute little shoaling fish, and among the best algae eaters for smaller tanks. They look like little plecos, and they stick to the glass and decorations in a similar way.

But, unlike plecos, they stay small and you can have a group of a dozen or so in a 20-gallon tank. However, I’d suggest keeping their numbers down around 6-8 to be sure they all have something to eat. Along those lines, you'll want to supplement their diet with algae wafers.

10. Harlequin Rasbora

These are pretty, vibrant fish that should be kept in a school of at least six. They’re peaceful community fish, and they’re vibrant orange/red colors look amazing in contrast to their black sides.

A well-planted 20-gallon tank with ten neons and ten harlequin rasboras would look amazing! It should be noted that all of these little fish I am recommending should be kept with other small, docile fish. To big fish they are lunch!

Critters for Your Aquarium

In addition to the fish listed above, you might consider some critters. When I say critters I mean animals like:


Apple and mystery snails are fun to watch. They’ll clean the algae in your tank and much on leftover fish food, and you’ll want to supplement their diet with algae wafers and veggies. (They may also eat your live plants, so they’re not the best choice for a planted tank.)

Note that these guys aren’t the same as the pest snails you may have in your tank, and they grow much larger. Count each snail as one fish for the purposes of stocking.


There are a few different types of shrimp to consider for your 20-gallon tank. Ghost shrimp are among the most common. They are inexpensive and have a low bio-load impact. As scavengers, they’ll help keep your tank clean, but be aware that some larger fish may see them as food.

You also might consider cherry shrimp, which are a little nicer to look at, or Amano shrimp who are excellent algae eaters.


African Dwarf Frogs are adorable little fully-aquatic amphibians, and tons of fun to keep. However, they can be a little tricky for novice fish keepers. They should not be kept with fish that will nip at them or try to eat them. They will not be able to compete with most fish for flake food, so be sure to include a variety of items such as sinking pellets and algae wafers.

These clever little guys can get out of your tank if they put their minds to it, so make sure you have a tight lid on. Finally, be careful not to confuse them with the African Clawed Frog, which grows much, much larger.

Big Fish for Your 20-Gallon Tank

If you want to keep large fish in your tank I really advise considering a 55-gallon or bigger. Twenty gallons give you more options than stocking a 10-gallon tank, but you still have limits.

Here are a few fish people often consider, as well as some to avoid:

  • Angelfish: I’d really rather see angels in a 55 or bigger, but you could keep a pair in a standard 20-gallon aquarium, as long as you don’t add many more fish. Angelfish get a lot bigger than many people realize, and as adults, they’ll eat small fish like tetras and guppies.
  • Gourami: Both angels and gouramis are semi-aggressive fish, so if you intend to keep either in a tank this small I wouldn’t include anything else aside from a few bottom feeders.
  • Plecostomus: There are no pleco species that are appropriate for a 20-gallon tank. Even smaller species, like rubber-lipped plecos, should be in a tank of at least 30 gallons. The common pleco has the potential to grow to a couple of feet and is only appropriate for truly huge aquariums.
  • Goldfish: You can keep one goldfish in your 20-gallon tank, and nothing else. Goldfish have unique care requirements. They can get very large, and it is a myth that they will grow to the size of their environment. Those that do end up stunted and sickly. They pollute water quickly and require slightly cooler water than tropical temps, and so should not be kept with tropical fish.
  • Silver Dollars, Bala Sharks, Tinfoil Barbs: These fish are shiny and tempting when they are juveniles, but they all grow much too large for your tank.

Good Luck With Your Aquarium!

I hope you aren’t disappointed after reading through his article. A 20-gallon aquarium offers many stocking options beyond a 10-gallon; however, they do have their limitations. If you want big fish or lots and lots of fish, you are probably better off upgrading to a 55-gallon tank or larger.

On the other hand, if you want to build a peaceful community tank with some pretty fish and maybe some nice plants, an aquarium this size may be all you need.

Good luck whatever you decide!

This article is accurate and true to the best of the author’s knowledge. It is not meant to substitute for diagnosis, prognosis, treatment, prescription, or formal and individualized advice from a veterinary medical professional. Animals exhibiting signs and symptoms of distress should be seen by a veterinarian immediately.

© 2018 Eric Dockett


Eric Dockett (author) from USA on June 06, 2020:

Hi Hi! I would not keep a pair of goldfish in a 25-gallon tank. I think the danios are a better option, in my opinion. Good luck!

Hi on June 05, 2020:

Hi can i have two goldfish in a 25 gallon or 6 glo fish tertras and 5 danios which option is better. Im probably gonna get a 25 gallon coffe table and i dont know which option is better. Im moving to a bigger house soon so thats why im gettin this tank. I already have more fish like comet goldfish,bettsas,corydoras and african dwarf frogs and snails in the tanks. I have many pets reptiles a dog, sanils should i get this tank.

Taya Garcia on March 21, 2020:

Alright. Thanks for the quick response! :)

Eric Dockett (author) from USA on March 21, 2020:

@Taya - I would consider adding the cories before adding more cardinals.

Taya Garcia on March 20, 2020:

I have 13 cardinal tetras, 2 mollies and a betta in a 20 gallon. Initially, I wanted to add 7 more cardinals, but I also wanted to buy some corydoras (probably a smaller species). Would you recommend adding any more fish from here? And if so, would it be okay to have 20 cardinals, 1 betta, 2 mollies, and 3 cories in a 20 gal?

Eric Dockett (author) from USA on January 26, 2020:

@Holly - Unless you are building another pond, I would try to re-home most of them. The 75 gal tank is a temporary solution at best.

Holly on January 25, 2020:

I have 23 Koi’s and goldfish. They were in a outside pond,but we had to move so now I have them in a 75 Gal tank. What should I do? Give the big ones away or just keep them? Thanks sooo much

Eric Dockett (author) from USA on December 09, 2019:

@John - You should not get an angelfish because, as you said, the angel will eat the neons when it gets bigger. Search for other, more compatible tank mates. Several are mentioned in this article. If you want angels you need bigger tank mates to go with them.

john on December 08, 2019:

i have neon tetras and i want angle fish but when they get big they eat then so wat should i do

Gaylord Robinson on October 31, 2019:

One important thing to mention is that the albino bristlenose CAN live in a 20-gallon tank, with it only reaching about 5 inches. It might not be optimal, but it certainly possible, size being considered.

Elizabeth Arner on October 03, 2019:

I think we should add snails to the tank as bottom feeders. Also we could add a beta to the tank for color. If the beta acts up at all I would be willing to take it home.

Eric Dockett (author) from USA on August 12, 2019:

@Su - Cories can do fine in a 10-gallon tank. The same suggestions apply as far as schooling, and be careful not to overstock your other fish. Good luck!

Su on August 12, 2019:

Hi there. I know that this is in no way related to the 20 gallon fish tank but, I was wondering if corydoras can live happily in a 10 gallon tank.. This is a very informative article for all fish enthusiasts. Thank you!

Eric Dockett (author) from USA on July 26, 2019: