How Many Guppies Can You Have in a 20 Gallon Tank?

9 min read

Guppies are a favorite fish for aquariums especially for people who are just beginning a new aquarium.

People are attracted to guppies because they are easy to take care of and are colorful fish, which is what an aquarium is for right? To look at colorful fish.

However, many beginning aquarists or people who aren’t familiar with guppies can put too many guppies in their tank, especially a 20-gallon tank which is the most popular size for an aquarium…

The rule for guppies is one gallon of water for each inch of length for your guppies. Male guppies can grow up to 1.5 inches while females can grow up to 2 inches so do the math. That’s 10 female guppies or 13 male guppies but if you have both sexes then you should have a ratio of 1 male per 2 females so if you mix sexes you can only have 3 males and 6 females per 20 gallons. If you account for tank decorations, plants, or filters, etc. you should minus 1-2 gallons from your total tank size depending on what you have in your tank.

You can’t just put as many guppies as you want in a fish tank, so let me explain in more detail how to know exactly how many guppies you can put in a 20-gallon tank…

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You Need to Keep Gender in Mind

Guppies may look the same to the untrained eye but…

Male guppies usually only reach 1.5 inches max while females can reach up to 2 inches in length when they are full grown.

The easiest way to tell them apart is that the male guppies are actually more colorful than female guppies so much so that you can tell them apart.

As stated above you can only have 3 males and 6 females to have the proper space in a mixed tank because of the 2 females for every male rule.

The good news is that 3 males and 6 females require 16.5 gallons of tank space so that leaves 3.5 gallons of extra space that you can use for your aquarium decorations, substrate, filter etc.

If you have all male guppies in your tank then you can have 13 male guppies because they grow to a max of 1.5 inches. 13 x 1.5 = 19.5 gallons. This only leaves ½ gallons of space for tank decorations, toys, filter etc. so you are probably safer going with 12 male guppies.

You want a little bit of extra space so you can provide your fish with more entertainment in the form of decorations, live plants, or toys.

If you have all female guppies, you can only have up to 10 female guppies because they grow to a max of 2 inches, so you need two gallons per fish. 10 times 2 is twenty gallons.

Of course, you might as well only have 9 female guppies because then you have a whole 2 gallons of room for plants, toys, filtration, etc.

If you have 10 female guppies then you can’t have anything else in your tank!

Why You Shouldn’t Have Too Many Guppies in A Tank

Don’t let the size of guppies fool you. Fishkeeping has become a tried-and-true profession over the years so don’t ignore sound advice from us and do something that can harm your guppies like keeping too many in a 20-gallon tank because you are ignorant. Here is what can happen:

Ammonia Levels Too High

You may already know that fish poop makes ammonia that reaches levels that can kill your fish. This is why the water in a fish tank has to be changed regularly, usually about once per week. You shouldn’t go past the recommended number of guppies in a fish tank, and you should buy a water testing kit.

If you continuously check your water, then you will know when and how often you need to change it. When you change the water it removes a large amount of water with ammonia in it and replaces it with fresh clean water.

Just make sure the water you put into the tank is treated for chlorine.

Too Much Stress

One of the problems with fish tanks is that you take the guppies out of their natural environment and cram them in a glass jail cell with confined space.

This is okay as long as they have enough room to swim around and get some exercise.

When you have more than the recommended number of guppies per gallon then they start running into each other because they are crowded, they become agitated, and this leads to stress.

Just like in humans, too much stress weakens the immune system, which can make your fish sick and even lead to death.

They can start fighting and their growth can even be stunted from too much stress.

Baby Guppies

On another note, if you keep male and female guppies together then they are more than likely going to have babies.

If you don’t want to deal with this then only keep all females or all males together.

If you do end up with babies then don’t panic. Just get in touch with your local pet store and ask them what to do. You can usually sell them to your local pet store or just donate them.

You can also sell them on craigslist or give them away.

If you want to keep the baby guppies then just buy another tank and get it ready.

Things You Need That Can Leave Less Space For Your Guppies


Make sure your guppy tank has a heater, especially if you live somewhere the weather is cold during winter. You may get away without a heater if you live somewhere like south texas but you should at least have a thermometer to be on the safe side.

In the rare instance, if you live somewhere hot, then you may need a water chiller to keep the temperature below 82 degrees.

A heater or chiller is usually inside the tank where it is more effective but you can get one that is built into to an hang on the back of your tank filter or a canister filter that can be placed underneath your tank.

Water Filter

Every fish tank should have a filter system unless you want to constantly change the water. As mentioned above, you need to change about 20-30 percent of the water weekly to keep ammonia levels down.

You only change a certain percentage of the water because you don’t want to lose the ‘beneficial bacteria’ that has already built up in your water.

Beneficial bacteria consume the bad bacteria that build up in your water. You lose some of the beneficial bacteria during a water change but most of the beneficial bacteria are in your filter, so you lose more ammonia and other bad stuff than beneficial bacteria during a water change. You can get an all-in-one filter that filters 3 things, or you can get an individual filter for each one.

You can get a hang on the back filter with all three filtrations built-in and takes less space away from your guppies.

You can also get a canister filter with all three types of filtration that fits underneath your aquarium.

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These three things are:


This is mainly all of the yucky leftover stuff in your tank like uneaten food and ammonia from the fish poop. This filter is where most of your beneficial bacteria will grow.


This filter is made to keep out large pieces of debris like gravel, leftover poop, sand, or any gunk that can build up in your fish tank and make your water cloudy


Most people use tap water that they add a chlorine treatment to for their aquarium water, but it can still have harmful chemicals in it.

This filter is usually carbon and will remove any chemicals that are harmful to your fish.


Guppies love to hide and play among plants. You can get live plants that will help filter the bad stuff out of your water or you can get fake plants that require less care than live plants.

Here are some of the best plants for guppies:

American waterweed

Java ferns


Guppy grass

Amazon sword




It is important to have plants for your guppies, so you are going to need some gravel or sand to root the plants in, even fake plants need something to hold them up.

You can use gravel or sand at the bottom of your tank.

Sand is better for live plants, and it depends on your preference as far as looks.

Gravel is harder to keep clean though because it is easier for debris to sink down in between the pieces of gravel.

With sand, you usually just have to lightly vacuum the top of it before a water change.

With gravel, you will have to make sure you work the top layer of gravel over pretty good and you may even have to take a lot of the gravel out when you do a monthly 50 percent water change.

Frequently Asked Questions

How many fish can I have in a 20-gallon tank?

The general rule for fish in a tank is 1 gallon for each inch of fish so you need to know what the maximum length is going to be for the type of fish in your tank. You also need to know what the maximum length is for each sex male or female for the type of fish in your tank.

Also, most aquarium websites online don’t mention that you should account for other things in your tank that can take up space like substrate, plants, decorations, heater/chiller, etc.

How many guppies should be kept together?

Guppies should be kept in at least a trio. If you are going to mix genders with guppies, then you need two have two females for each male guppy.

What size tank do I need for 2 guppies?

Guppies should be kept in at least a trio but if you are going to try to keep two guppies, here are some things you should know.

Male guppies grow to an inch and a half, so you will need at least 3 gallons of water according to the gallon per inch fish rule. If you have any decorations, substrate, filtration, plants, etc. you will need extra space. Accordingly, for two male guppies, you will need at least 4.5 gallons but the more the merrier, so you might as well go with five gallons.

It also isn’t recommended to keep fewer than 3 males because of bullying. Guppies breed a lot so they can be aggressive if there are no females. It is also recommended to keep at least 6 male guppies if they are all male so one male isn’t singled out. You should also have plenty of hiding spaces for the weakest male to hide.

If you have two female guppies, they grow up to two inches, so according to the gallon per inch fish rule, you will need four gallons. However, if you have any substrate, plants, decorations, filtration, etc. you will need more space so you might as well go with five gallons.

If you mix genders, you need two females for every male guppy so you must have 3. According to the gallon per inch rule, you would need at least five gallons since females grow to two inches and males grow to 1.5 inches. If you have any substrate, plants, decorations, filtration, etc. in the tank you will need extra space, so you will need 5.5 to 6 gallons.

Can you keep three male guppies together?

Yes, it is recommended to keep at least three guppies because they are shoaling fish which means they prefer to live in groups.

With males, it is recommended to keep at least six because they breed a lot and without females, they can be aggressive and bully each other. With at least six, the weakest guppie is less likely to be singled out.

You should also have plenty of hiding places in case of bullying, so the weakest guppy will have a place to hide.

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Pet Aquariums

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