How Many Guppies in a 20 Gallon Tank? (And How To Calculate)

If you’re wondering how many guppies you can keep in a 20 gallon tank then you’ve found the right article!

In this article not only will you find out just how many you can keep, you’ll also learn how to calculate how many fish you can keep in any tank in the future, whether the gender of your guppies matter, and much more!

So keep reading to find out everything you need to know

How Many Guppies in a 20 Gallon Tank? 

A 20-gallon tank can hold around 10-12 female guppies of 2-2.5″ in size or 12-14 male guppies of 1.4-1.6″ in size. If you planned on keeping a mix of both, then you should add no more than 12, keeping a 1:2 male to female ratio.

How Do You Calculate How Many Guppies You Can Keep in a 20 Gallon Tank? 

Do not let their petite size fool you; it’s a mistake to add as many guppies as you want into a fish tank. You need to bear in mind guppies need to have a lot of room to swim, and adding too many fish can cause various problems, which will be discussed below.

Another thing to keep in mind is that no tank is going to be full from top to bottom due to other pieces of equipment. Therefore, you need to be careful when deciding on how many guppies to add to your aquarium. 

The general formula to calculate how many guppies you can add to a 20 gallon tank is one gallon of water per one inch of guppy. In other words, a two-inch guppy needs two gallons of water.

Although it’s entirely possible to keep 12 guppies in a fish tank I wouldn’t recommend it if you’re new to fishkeeping. If you haven’t had fish, especially guppies, before, it’s not a good idea to start with a high number.

With more fish in a tank comes more chance for water parameters to change drastically. So keeping less guppies will give you more leeway for mistakes.

When choosing how many guppies can live in your 20 gallon tank the priority should always be to ensure your guppies have a comfortable living environment, oxygenated water, and suitable temperature. This is why it’s important to remember that the number of guppies in your fish tank isn’t the only thing that matters, but also how much equipment you’re filling your tank with.

Does Gender Matter? 

To inexperienced people, all guppies (and other fish) look the same, and there are no visible differences between males and females. Not true, though! Male and female guppies have a lot of differences, and gender is crucial when calculating how many to hold in a tank.  

Generally speaking, males have longer and slender bodies, but female guppies look rounder and curvier. That said, females tend to be bigger than males. Male guppies tend to grow up to 1.5 inches long, while females can reach two or even 2.5 inches in size. 

As you can guess as well, males are a lot more colorful than females, as they use their color and shimmering tail to attract the females attention.

Because of the difference in size, you’ll be able to keep more male guppies than female guppies in a tank, as males tend to be smaller.

What Will Happen If You Add Too Many Guppies? 

Due to their small size, it’s easy to think nothing will happen if you add as many guppies as you want into your fish tank. However, an overcrowded aquarium is definitely not a good idea, which is why you should stick to the calculation mentioned earlier. Adding too many guppies into a fish tank can pave the way to a wide range of problems such as: 

Too Much Ammonia

Guppies produce ammonia, which can be toxic for them. Which is one of the reasons keeping a filter in your guppies tank is so essential. Every filter has its limit, so when the tank is overcrowded, it may not work properly to filter the water. When water isn’t filtered, oxygenated, and clean, then your guppies may begin to suffer from ammonia poisoning as well as other illnesses.  

Weakened Immune System

When there are too many guppies in your tank, the over crowdedness mixed with the excess waste being created will stress your guppy out and make them much more susceptible to a whole range of diseases.

Increased Stress

when you decide to have a fish tank, the goal is to recreate the natural environment of your fish as much as possible. By overcrowding your tank, not only will you be making the tank less like their natural environment, but you’ll also be making the parameters dangerous to your guppies too.


With the increased stress of guppies in the overcrowded fish tank, there may also come aggression. Guppies may feel frustrated by all the competitors in the aquarium with them, which can result in fighting, bullying and aggression.

 (Find out more about guppy fighting and aggression.)

Developmental Issues 

Guppies aren’t big in size, but too many fish in the tank may impair the growth of young fish. Developmental problems directly harm their quality of life and may shorten their lifespan.  On top of this, guppies are livebearers, so the more fish, the more inbreeding, and the more likely guppies will be born with birth defects.

(Find out how to care for your guppy fry.)

What Effect Will Breeding Have? 

Guppies breed like crazy, and this is an important factor to bear in mind when deciding how many fish to keep in the tank. The reality is that if you have a male and female guppy in a fish tank, they’ll breed regardless of whether you want that or not.

Female guppies can have multiple pregnancies from only one fertilization over a certain period. This can lead to the overcrowded fish tank, which was discussed above. 

Therefore, when deciding how many guppies should be in a 20-gallon fish tank, you also need to take into consideration their gender and the fact they breed a lot. The fish tank has to accommodate your guppies perfectly, so if you’re not ready for them to breed all the time, then you may want to opt for males or females only.

What Tank Mates Can You Keep With Guppies In A 20 Gallon Tank?

If you’re not sure you just want to keep guppies on their own, then there are plenty of tank mates you can also keep with your guppies as well.

Some great choices of tank mate for your guppies include:

  • Neon Tetras
  • Bronze Corydoras
  • Pygmy Corydoras
  • Mollies
  • Platies
  • Harlequin Rasboras
  • Amano Shrimp
  • Cherry Shrimp
  • Malaysian Trumpet Snails
  • Ramshorn Snails
  • African Dwarf Frogs
  • Chili Rasboras

Check out this article if you want a more in-depth explanation of the tank mates that can live with guppies.

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How Many Mollies And Guppies In A 20 Gallon Tank?

In a 20 Gallon tank you should only keep 3 full size mollies and 3 full size guppies together. If you want more you should get them while they’re still small, however, remember that once they grow, you’ll need to move some to another tank.

How Many Guppies And Neon Tetras In A 20 Gallon Tank?

You need a minimum of 6 neon tetras in a tank, and a minimum of 3 guppies following a 1:2 male to female ratio. However, after that you can add more of both to your tank until you reach 20 inches of fish.


The number of guppies in a fish tank is crucial. You need to take into account their size and gender when deciding on how many guppies should be in a 20-gallon tank. Males and females come in different sizes, so the same rule can’t apply to both. For example, you can fit more males than females into a tank, but most people prefer to mix them up.  

When adding both genders, always bear in mind these fish breed a lot, so you need to be careful if you want to prevent the overcrowding problem. It’s also important to emphasize that proper care is crucial. Guppies need a warm temperature of the water and hate stagnant water in the fish tank, so make sure to recreate their natural environment the best way you can. 

(Find out how many guppies can stay in a 5-gallon and 10-gallon tank.)