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How Many Cardinal Tetras Can You Have In a 20 Gallon Tank? Vet-Approved Facts & FAQs

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By Lindsey Stanton

cardinal tetra

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Cardinal tetras (Paracheirodon axelrodi), sometimes referred to as “red neon tetras”, are indeed some very small fish, ones that grow to a maximum length of 2 inches. However, just because they are small does not mean that you can cram them into a miniature tank.

Many people may wonder, “how many cardinal tetras in a 20-gallon tank?”. You can comfortably house 10–15 cardinal tetras in a standard 20-gallon long aquarium, assuming you’re on top of your weekly partial water changes.

Let’s take a closer look at cardinal tetra tank size requirements, housing requirements, and more.

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How Many Cardinal Tetras Should Be Kept Together?

Cardinal tetras are schooling fish, which means that they do not like to be kept alone. They enjoy the safety in numbers, which is their natural way of staying safe from predators.

At a bare minimum, you should keep at least 6–8 cardinal tetras together. A school of 10 cardinal tetras is a great size to go with if you want to make them feel at home. These fish tend to swim with more confidence and are more lively when they’re in higher numbers.

Cardinal tetra
Image Credit: InsectWorld, Shutterstock

Minimum Tank Size For Cardinal Tetras

A school of around 6–8 Cardinal tetras would need an aquarium of at least 10 gallons as a bare minimum, though they would probably be better off in a 15-gallon aquarium. Beyond this, a rough guide is to add around 1–2 gallons of water per additional fish. This is a rough guideline for cardinal tetras only, and doesn’t apply to other fish.

For a school of 10–15 cardinal tetras, a 20-gallon long aquarium (30” x 12” x 12”) would be recommended. Again, it’s best to spring for more space whenever possible.

Cardinal Tetra Housing Requirements

The size of the tank is not the only important thing that needs to be considered when you are looking to keep some cardinal tetras in an aquarium.

Here we have a complete list of cardinal tetra housing requirements, so you know exactly what you are getting into.

Water Temperature

Aquarium with plants and a thermometer on the windshield
Image Credit: Lapis2380, Shutterstock

Cardinal tetras are warm water tropical fish that need their water to be between 24–30 °C (75–86 °F).

It’s best to use a water heater to maintain your aquarium at a steady temperature. It is best to keep your aquarium in a room that’s colder than the temperature requirement of your fish. This ensures that your heater will keep your water temperature at a consistent preset value. If the room is warmer than the setting on the heater, it wouldn’t be of much use.

Water Hardness

Cardinal tetras are relatively adaptable to a wide range of water hardness values, and any value between 1–15 dH is considered appropriate, as long as it remains consistent. For most fish keepers, it’s best not to try adjusting the water hardness of the water source used in their aquariums.

Water pH

Cardinal tetras are able to adjust and thrive in pH values ranging from 5.5 to 7.5. They do prefer slightly acidic water (below 7.0), however, once again, it’s best to stick with consistency rather than trying to constantly attain a specific pH value for your aquarium, as fluctuations in pH will undoubtedly stress your fish.

Man determining water pH by comparing the color of liquid in testing vials with attached color scale
Image Credit: tetiana_u, Shutterstock

Filtration

Cardinal tetras are otherwise fairly hardy, but you will still need to get them a decent filter.

If you have a 20-gallon tank, you should aim for a filter that can handle at least 60 gallons of water per hour. This will ensure that the tetra tank will be as clean as can be.

Remember that you will want to invest in an aquarium filter that engages in all 3 major forms of filtration, including mechanical, biological, and chemical filtration.

On a side note, cardinal tetras, while they can handle a bit of flow, do not like strong currents, so you do need to keep the flow rate on the lower end of things.

Lighting

Cardinal tetras require a very basic aquarium light. They actually don’t like too much light, so something soft that is not too bright will be best. If you have live plants, then your lighting choices should be catered to your plants’ needs.

Many people choose to use some floating plants in their cardinal tetra tanks in order to provide a bit of cover from the lighting above.

Substrate

When it comes to the substrate, you will want to go with some really fine gravel or aquarium sand, with fine gravel probably being the better option.

In the wild, cardinal tetras live in river basins that are both rocky and sandy, complete with lots of vegetation.

About an inch or an inch and a half of fine and smooth aquarium gravel will be perfect for a cardinal tetra tank, especially when it comes to rooting plants.

aquarist preparing substrate in aquarium
Image Credit: rodimov, Shutterstock

Plants

Some great plants for your cardinal tetra tank include Amazon swords, Anubias nana, and Java ferns, as they are easy to care for, provide your tetras with some cover and privacy, and if cared for right, won’t take up too much space in the tank.

Cardinal tetras do like a decent amount of plant life in their tanks, but they also prefer the center to be open for swimming.

So, this means that any plants you put in the tank should ideally (but not necessarily) be in the background and around the edges.

Rocks & Decorations

aquarium castle
Image Credit: Krysja, Shutterstock

Cardinal tetras do enjoy some hollow driftwood and little hollow aquarium castles which they can swim through and hide in.

Other than that, you really don’t need any special decorations for a cardinal tetra tank, and there’s a lot of freedom of choice here.

Tank Mates

Cardinal tetras are very peaceful fish that won’t hurt others, so any tank mates that are roughly the same size and are also peaceful will do fine here.

However, fish that are larger, may attack your tetras, or even eat them, should be avoided at all costs.

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FAQ

Are cardinal tetras hard to keep?

If they’re cared for properly, cardinal tetras aren’t that difficult to keep successfully.

How many cardinal tetras in a 40-gallon tank?

You could potentially fit up to 30 cardinal tetras into a 40-gallon tank, provided you have no other fish in the tank.

Will neon tetras school with cardinals?

No, cardinal tetras and neon tetras will not school together, despite them being closely related to each other. However, they are considered compatible with each other in terms of tank mates. They may swim near each other, but don’t tend to school together in a coordinated fashion.

Cardinal tetra
Image Credit: chonlasub woravichan, Shutterstock

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Final Thoughts

Cardinal tetras are small, but they do need plenty of swimming space to thrive. For a 20-gallon aquarium, 10–15 cardinal tetras would be the maximum, assuming you’re on top of your weekly water changes and have no other fish in the tank. As always, with fishkeeping, bigger tends to be better, so springing for a larger tank is definitely recommended whenever possible.


Featured Image Credit: Roberto Dani, Shutterstock

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