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You wouldn't want to live in a house with too many people, would you? You and your housemates are bound to clash at some point. It's the same with fishes in a tank.

Fishes fighting for space in your tank could lead to unnecessary stress, poor health, and even death. 

20-gallon tanks are sufficient for displaying the beauty of the underwater world of fishes. However, a 20-gallon tank is a nano tank. If you put too many fishes in it, the water becomes dirty quickly, and the fishes end up fighting for resources.

The biggest problem you will have is knowing how many fishes to put in the tank. Also, since fishes sizes vary, you may find it difficult to understand those that will thrive and the balance to keep with other fishes.

If you are thinking of getting a 20-gallon fish tank, this guide will be a helpful resource to get you started. Aquarium owners battling with overcrowding in their tank will also find the info in this article useful.

We'll provide you with all you need to know about setting up your 20-gallon tank with the right amount of fish. We'll also suggest possible fish species that will be suitable for it.

Let's begin with some information about setting up 20-gallon fish tanks.

Guidelines to Planning Your 20-Gallon Tank

Smaller tanks are more challenging to set up and care for than bigger tanks because you might need to change the water many times a week. However, technology has made this process easier, thereby making smaller tanks more popular.

Set the Nitrogen Cycle

Water quality in your tank is dependent on the colonies of good aquarium bacteria in your substrate and filter. Bacteria turn harmful ammonia from fish waste into nitrates that plants use for food. Hence, it's a crucial part of the nitrogen cycle. 

Toxin build-up in a small tank happens quickly, so you need to make sure your water quality is good at all times. Give your plants a couple of months to establish themselves before introducing fishes into the tank. This is to set the balance and prevent ammonia spikes.

Get a Filtration System

It is necessary to get a filter for your 20-gallon tank because it will help clean the water and remove excess ammonia. Aquatic plants have more to gain with the water quality than many fish species. You need a high-quality filtration device (either HOB, internal, or submersible) to prevent any sudden changes in your water quality.

Your filter’s quality will also determine how healthy your fish will be and how frequently you need to change your tank’s water.

Add Plants and Décor

No matter their sizes, fish tanks should resemble fishes’ natural habitat such as lakes or rivers. Apart from the live plants, decoration pieces such as logs, sticks, rocks, and other fishes’ natural materials should mimic the fishes’ natural environment in your tank. 

Get LED Lighting

Aquarium LEDs are good lighting systems for 20-gallon tanks because they are small and won’t take up unnecessary space in the tank. They also deliver great results.

Putting a light in your tank isn’t compulsory, but it will give you more options when choosing vegetation. Be sure to look for a quality and portable LED light.

Set the Temperature by Adding a Water Heater 

Putting a water heater in your tank is dependent on the temperature range of your house and the type of aquatic animals you intend to get. Some cold-water tiny fishes will be just fine in the water at room temperature. Other fishes will be stressed if the temperature changes more than a little in a couple of hours.

However, a water heater in a 20-gallon tank is strongly recommended to prevent any sudden change that could stress your fish or make them sick. Many small fishes prefer warm water, so you will have many more choices when you use a heater to maintain the temperature.

Tips for Choosing Fish in a 20-Gallon Fish Tank 

A popular rule for adding fish in an aquarium state of all your fish’s collective length shouldn't exceed the tank’s number of gallons. In the case of the 20-gallon tank, that means 20 inches of fish length.

While the one-inch per gallon rule for selecting fish applies, there are still other factors to consider when choosing fish for your 20-gallon tank. Here are the most important:

  • Choose fish that likes planted tanks with a lot of spots for hiding.
  • Avoid fishes that are territorial or aggressive and choose peaceful ones.
  • Look for fishes that won’t remove your plants just for the fun of it.
  • Select fishes that are okay with being alone or in small groups.
  • Go for small fishes that do not produce many biowastes, no longer than 5-inches in length when they're fully mature.
  • Ensure the temperature and pH requirements of all the fish’s match. Tropical fishes can adjust much easier than cold-blooded fish. Goldfish is probably the most versatile.

Top 7 Best Fishes for 20-Gallon Tank 


Guppies are small but extremely active fishes. They have different color patterns on their body and are sexually dimorphic. You will rarely see a guppy fish engaged in any form of physical altercation, except they have predator fishes as tankmates. The males are generally smaller than the females and often show off their bright colors as they move around the tank.

Fancy Guppies are a go-to choice for a lot of small aquariums, especially for beginners. They are omnivores that average about 1-2.5 inches in length. The best temperature range for a guppy is 75-82oF.

Who is it for?

Fish-keeping beginners and those who want community tanks should get this one.

Otocinclus Catfish

Otocinclus catfishes are known to eat up all the algae in the tank. They’re a great cleanup crew because they eat all the decaying vegetation. They make great inhabitants of planted tanks. 

The otocinclus catfish grows up to 2 inches and will be comfortable in a temperature range of 72 -82oF. They tend to school if you have them in groups of 4-6.

Betta Fish

These fishes are one of the best choices you can make for your 20-gallon tank, especially if you’re new to fish-keeping. Although male bettas are aggressive and fight with each other, they are gentler when they're solitary, especially with same-size small fin-fishes. They can grow to lengths of 5.5 inches, depending on the variety you have.

The females grow up to 3.5 inches, and they can be kept together in groups of three to avoid competition. You could also go the route of making it a solitary fish, which also works, especially if it's with other peaceful community fishes. Betta fishes thrive in temperatures of 75-86oF.

What to remember?

Betta fishes grow quite long compared to other species on this list. We recommend getting the females over the males.

Neon Tetra

These wildly popular and inexpensive fishes will be a great addition to your 20-gallon tank. They have colors of red, silver, and blue, so they're easy to spot. Neon tetras are also sensitive to slight changes in the water quality of your tank, and you should put them a couple of months after you have cycled your tank.

Neon tetras grow to about 1-1.5 inches and thrive in temperatures from 70 to 81oF. They can school, but only in larger groups of at least 15.

Here’s the deal:

These fishes require constant care and a strict maintenance schedule.

Pygmy Corydoras

Pygmy Corydoras can be classified as bottom-dwelling or mid-level fishes. They have visible black markings on silver, and the school if you keep them in groups ranging from 4 to 8. You will have almost no challenges taking care of these fishes, and they tend to excel in planted tanks. You can get them as solitary fishes or put them in mixed communities with no problem. They get along well with other fishes.

Pygmy Corydoras, also known as Pygmy catfish, can grow in length to 1 inch, and they require temperatures of 72-79oF to thrive.

What’s the deal with this fish?

A great choice for community tanks and otherwise, and you will enjoy watching them play in your tank.

Licorice Gourami

You can easily spot these fishes from a distance because of their distinct blue, brown, and white colors. Licorice Gourami is timider than their cousins – Sparkling Gourami, which makes them gentle and usually well-reserved. They tend to fare better in groups or pairs. We would advise you to put only one male in your tank to prevent competition. 

Licorice Gourami is an omnivore and grows to lengths of 1.2-1.6 inches. You should have a minimum tank size of 5 gallons for a pair, and they thrive in temperatures of 72 to 82oF.

Here’s the downside:

The main challenge you might have with this species is they are picky eaters, so they may not be ideal candidates for community aquariums.

Zebra Danio

If you intend to have a community aquarium, you can do no wrong with zebra danios. They are one of the most researched fish species globally, and as their name suggests, they have black stripes on their silver color. If you would like other color morphs and albino variety, you can also get them at your local fish shop or online. These lively fishes are happiest when you place them in groups of at least 15.

Zebra Danios are omnivores that grow in length to about 2 inches. You need a minimum tank of 10 gallons to have them, and their temperature range is 64-77oF.

You might be wondering:

Zebra Danios are popular fishes but will only be at their best in a community tank.


At What Point Does an Aquarium Become Overpopulated?

Overpopulation in a tank occurs when the tank’s resources can no longer cater to all occupants. The fishes start becoming stressed or aggressive. When space in the tank becomes overcrowded, it will be difficult for fishes to be comfortable, making them aggressive. 

If you notice that you need to change the water in the aquarium more frequently, then your tank might have become overpopulated. One of the main causes of this is not accounting for the fishes becoming adults. You should also consider that the fishes you put in your tank might give birth and further populate the tank when you are selecting.

What are the Dangers of an Overpopulated Aquarium?

The water will get dirty faster, putting more pressure on the filter to keep up with tank’s the increasing needs. Algae growth in the tank will increase exponentially as the water quality of the tank drops. 

The food in the tank will also not be sufficient for the fishes, leading to more competition. Some fishes may overeat because they’re scared that they’ll not get enough food. Excessive competition in the tank can cause some fishes to be more territorial and predatory. All of this can stress the fishes resulting in several illnesses or even death.


The number of fishes you can put in your 20-gallon tank is dependent on the type of fishes you like, plants, and décor in your tank. 

However, there are some factors to look out for when selecting fishes for a 20-gallon aquarium, such as avoiding territorial fishes, fishes that produce many biowastes, and others, as explained above.

Whatever the case may be, taking care of fishes is a big responsibility. So you have to do extensive research on any species you decide to get. You need to make sure the tank can serve as a suitable habitat while providing your fish’s needs.

About the author 


My name is Eric. I'm the owner of snugaquarium.net and a writer with a passion for aquariums and fish-keeping. I love to watch the three different species of freshwater fish floating around in my homemade aquarium in my spare time.

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