Best Fish For a 20 Gallon Tank

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A 20 gallon tank is a nice size tank that allows for a good amount of fish but you need to resist the temptation to overstock this tank.

So, exactly which are the best fish for a 20 gallon tank?

The general rule of thumb is an inch of fish per gallon of water.

This is the adult size of the fish and not when they are youngsters.

Too many fish can cause them to be stressed and lead to an outbreak of disease.

So it is really important that you do not overstock but rather under stock your tank.

Although your tank is classified as a 20 gallon tank, it will not actually hold that amount of water.

The decorations, gravel, heater, plants and internal filter will all reduce the amount of water that actually sits in your tank.

The shape of the tank is also important. A long tank will be able to hold more fish then a tall narrow one. The larger the surface area of the tank, the more fish it can support.

Size Considerations

Make sure that you research any fish that you are considering before buying them.

You need to know what there final adult size will be to accurately determine how many fish you can stock your tank with.

Specific Needs

While doing your research also note down the temperament and behavior of the fish.

Some fish will only be happy in a school of 6 or more of it’s own kind while others are aggressive and do not do well with other fish tank mates.

Fast swimming fish will need more space than slower swimmers because they are more active.

Larger bodied fish like Goldfish create a lot more waste and need bigger tanks.

So there is a lot to consider before even stepping into a pet store or buying online.

best fish for a 20 gallon tank

Filtration Matters

Filtration plays a large part in how many fish your tank can hold.

A filter should turn around the total volume of water 4 times per hour.

This means that a 20 gallon tank needs a filter rated at 80 gallons per hour at the bear minimum. It is always safer and my recommendation to go for a higher volume filter.

Best Fish for 20 Gallon Tank

  • Livebearers – guppies, platies and swordtails
  • Tetras – Neon, Cardinal, Glowlight, Rummynose and Black Phantom Tetra
  • Rasboras –  Harlequin, Red-lined and Pygmy Rasboras
  • Danios – Zebra and Celestial Pearl Danio
  • Cory Catfish – Panda, Sterbai, Juli, Bronze and Pepper
  • Barbs – Cherry Barbs (Placid) and Tiger Barbs (Aggressive)
  • Dwarf Gourami – Honey and normal dwarf gourami
  • Cichlids – German Blue Ram, Apistogramma and Kribensis (Some are aggressive)
  • Bettas – Only keep one male per tank or you can keep several females

Centerpiece fish for 20 gallon tank

 A centerpiece fish draws attention, stands out through either color or size, and also gets along with the other fish in the community tank. 

My top 5 choices (in no particular order) are:


An angelfish will create a great focal point. They do get quite large so you will only be able to have one in your tank.

Dwarf Gourami

There are various colored varieties of dwarf gourami and they are each stunning in there own right. They can get aggressive when they are in breeding mode but normally leave the other fish alone.
Dwarf Gourami


Another striking fish that can be a centerpiece fish for your tank. Only have one male and one female as these fish are on the larger size.

Bolivian Ram 

An often overlooked fish because they can be dull in the pet store. They only display there true colors when they are not stressed and are happy. They are easy to care for and generally peaceful.
Bolivian Ram

Betta Fish

Betta fish have great personalities and can be taught to eat out of your hand. Only one male can be kept at a time as they will kill any other male betta fish. With there long flowing fins and attractive colors they make awesome centerpiece fish.
betta fish

Big fish for 20 gallon tank

Unfortunately, a 20 gallon tank is not big enough for large fish.

Goldfish need at least 30 gallons to thrive as they are messy fish.

The biggest fish I would even consider would be the Angelfish and Dwarf Gourami.

How many guppies in a 20 gallon tank


Guppies do well in a 20 gallon tank.

You can just have male guppies so that there is no breeding involved or you can get males and females.

The males are the most colorful.

If you do get both sexes then make sure that you get double females to males as the males really pester the females and never seem to leave them alone.

In a 20 gallon tank you can house around 10 to 12 guppies.

How many neon tetras in a 20 gallon tank

Neon Tetras are very popular and are readily available.

They are very active and like to swim throughout the whole tank. Neon tetras crave being in a school, and their beauty is in their schooling.

They can reach 2 inches when fully grown so a 20 gallon tank should be able to house around 10 neon tetras quite comfortably.

How many mollies in a 20 gallon tank

The short-finned molly is relatively small and grows to a length of 4 inches making them suitable for this sized tank.

Unfortunately, the sailfin mollies which grow to a maximum size of 6 inches, should have at least a 29 gallon tank to accommodate there size.

The short-finned molly needs 5 gallons of water per fish so a 20 gallon tank can house 4 mollies. I would suggest getting only 3 and having one male and two females.

How many platies in a 20 gallon tank

platies platy

Platies are peaceful, easy-to-care for, and ideal for community aquariums.

They are hardy fish that are much happier when kept in a small group.

Males are always slightly smaller than the females and should be kept in a 1 to 2 ratio.

Popular varieties include Redtail, Blue Moon, Yellowtail, Sunset, Rainbow, Hawaii. Mickey-Mouse and the Marigold Platy.

You should be able to house at least 5 platies in your tank – 3 females and 2 males.

How many dwarf gouramis in a 20 gallon tank

If you are going for the original dwarf gourami or a color variant of it, then I suggest a single pair in a 20 gallon tank.

If you are thinking of adding the honey gourami then you can get away with 2 females and one male. This is because honey gourami are much more peaceful then the dwarf gourami.

Most gouramis are surface oriented, so have tall and floating plants at the surface to help make them feel at home.

They will be less stressed and show their best colors in a well decorated aquarium. The aquarium must have a lid as these fish tend to jump.

Red Flame Dwarf Gourami


As you can see, there are lots of options when stocking your 20 gallon tank.

You will need to first cycle your tank and once done then add the fish slowly over the course of a few months.

This will prevent your tank from being overwhelmed by fish waste and uneaten fish food.

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Irma Bense is the founder of She has over 42 years of experience in keeping fish.She has kept both freshwater and saltwater fish through the years.She has extensive knowledge in breeding numerous species of fish from Show Guppies, Cory Catfish, German Blue Rams to Apistogrammas.You can learn more about Irma on the 'About' page.

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