How Many Guppies in a 20 Gallon Tank?

Guppies are a small rainbow like fish and a popular freshwater aquarium species. Their appearance can make them fascinating to watch inside your tank.

Despite being smaller in size, they require a sufficient amount of room to survive. You must not fill your tank with a large volume of guppies simply because they appear smaller so you don’t feel as though they need so much room. This simply is not the case. 

The size of the tank that is required to house guppies can be determined by a ratio. Ideally, your tank should have 1 guppy per 2 gallons of water. If you have a 20 gallon tank, it is advised to keep 10 guppies inside.

It is also important to consider whether they are male or female guppies as this can affect the ratio of guppies to the gallons of water. If you are likely to have a mix of both genders, they should be kept at a 2:1 ratio.

Many factors can determine how many guppies can be kept in a 20 gallon fish tank. We have highlighted these below. 

The Gender

First and foremost, it is important to consider the gender of the guppies that are going to be kept inside the tank. There are distinct differences in the appearance of both genders.

Male guppies are known to be smaller and thinner with longer bodies. Female guppies grow considerably bigger and have rounder bodies. 

Due to the differences in the size of male and female guppies, it is important to note that having a mixture of both means you are going to be able to have fewer guppies than if you were to select a single-gender. 

The difference in size also means that the amount of guppies that can be contained in a 20 gallon tank is going to vary depending on the one that you choose. 

The Size

As we have mentioned the size of your guppies is going to be determined by their gender. However, it is also important to consider the differences between the size of the guppies at the time of purchase to when they are fully grown.

By 6 months old, the majority of guppies have reached adulthood and have grown to their full capacity. Male guppies tend to measure around 1.8” when fully grown. Females however tend to measure between 2 to 2.5” when they have fully grown.

Therefore, they aren’t going to be the same size from the time of purchase for the duration of the time that you have them. Purchasing a tonne of baby guppies because of their smaller size may see you with a tank that is overrun when they begin to reach adulthood. 


Of course, opting for a mixture of both males and females increases the chances of baby guppies. In fact, guppies are known to be fast breeders. Unlike many species of fish, guppies are livebearers.

They don’t come from eggs, they are born alive. Whether you intend for your guppies to breed or not, keeping a mixture of both presents this problem and if you already have a fish tank that is pretty full, the presence of babies can soon see it become overrun. 

There are many steps that you can take to prevent this if you do not wish to be presented with the regular issue of guppies breeding. The first and perhaps most obvious is to keep your male and female guppies separated.

This is going to require the need for an additional tank so it is worth considering this at the time of purchase to assess whether it would be beneficial to opt for a single breed.

If you would still like to keep a mixture of both options, another option is to ensure that the volume of males is higher than that of females. While they are still likely to breed, it is less likely to become as regular an issue.

Maintaining the Tank

The regularity of the maintenance required depends on the volume of fish inside your tank. A tank with a lot of fish is likely to require cleaning on a weekly or fortnightly basis. It also depends on how dirty your fish are.

It is also important to note that a tank with too many fish is going to be much more difficult to clean too. You don’t want to be attempting to clean your tank only to be challenged by a large amount of fish that make this process increasingly difficult. 

Having the correct amount of fish inside your tank, can make maintenance and cleaning a straightforward process.

Issues with Having Too Many Guppies

Having too many guppies inside your fish tank can result in multiple issues. Ammonia is produced by fish, however, too much can soon become toxic for them.

Some filters struggle to function properly inside tanks that are overrun. The inability for the filter to function can result in the buildup of ammonia which can affect the health of your guppies.

This can also affect the immune system of your guppies making them weaker as a result. It is also likely that a tank that is overrun with guppies is going to result in stress and possible aggression.

Not only are the guppies going to be restricted in movement, but they may also become competitive due to the volume of fish that they are contending with. 

Final Thoughts

There isn’t a definitive answer as to how many guppies can be kept inside a 20 gallon fish tank as this is dependent upon a range of factors. If you are considering purchasing guppies it is important to consider the gender and whether you want to opt for males or females or a mixture of both.

Remember that a mixture of both means that you will need to have a smaller amount inside the tank. Also, it is important to note that uncontrollable breading is a common issue with guppies.

As they breed at such a fast pace this can soon develop into issues with an overrun tank. Before getting your guppies it is important to take the correct steps to ensure that overbreeding isn’t an issue.