Hepper is reader-supported. When you buy via links on our site, we may earn an affiliate commission at no cost to you. Learn more.

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

Lindsey Stanton Profile Picture

By Lindsey Stanton

neon tetra

Cardinal tetras, otherwise known as neon red 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 ask us, “how many cardinal tetras in a 20-gallon tank?”. Each cardinal tetra requires a minimum of 2 gallons of space so you can house between 5 to 10 in a 20-gallon tank.

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

divider seashells

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 five 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.

Cardinal tetra
Image Credit: InsectWorld, Shutterstock

Minimum Tank Size For Cardinal Tetras

The minimum tank size for cardinal tetras is about 2 gallons per fish. The rule of thumb is that each inch of fish needs a gallon of space, and seeing as cardinal tetras grow to around 2 inches in length, each fish needs 2 gallons.

This means that a school of five cardinal tetras would need a tank of no less than 10 gallons, and a school of 10 fish would need 20 gallons.

Now, with that being said, the 1 gallon per inch of fish rule is the bare minimum, but the ideal tank size is going to be twice that, so 2 gallons of tank space per inch of fish.

Remember you also need to consider other fish you are or planning to house as tank mates, plants, filters, etc., but as always, the bigger tank you can get, the better.

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

Image Credit: Matteo Fusco, Unsplash

Cardinal tetras are warm water tropical fish that need their water to be between 73 and 80 degrees Fahrenheit.

This means that if you live somewhere the ambient temperature regularly drops below 73 degrees, you will most likely need to purchase an aquarium heater.

Water Hardness

One thing to note about cardinal tetras is that they cannot tolerate hard water, not in the least. These fish require the water to be very soft, with a KH level between 2 and 6.

This is very low, and chances are fairly high that you will require some sort of water softener to maintain such soft aquarium water.

Water pH

Cardinal tetras prefer their water to be on the slightly acidic side of things, with a pH level between 5.5 and 7.0.

Depending on your aquarium setup, you may need to use some sort of artificial pH reduction liquid to bring the acidity down to an acceptable level.

Hands holding high nitrite or ammonia test in front of freshwater aquarium
Image Credit: Ladanifer, Shutterstock


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, they do not like strong currents, so you do need to keep the flow rate on the lower end of things.


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.

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.


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


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 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.

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.

fish divider


Are cardinal tetras hard to keep?

No, in fact, cardinal tetras are some of the easiest to care for fish out there.

They are very hardy, they aren’t picky eaters, and as long as you keep the water clean, you should not see any health issues.

How many cardinal tetras in a 40-gallon tank?

You could potentially fit up to 20 cardinal tetras into a 40-gallon tank, although a school of 10-15 tetras would be ideal for this, just so they have enough space.

Will neon tetras school with Cardinals?

Yes, cardinal tetras and neon tetras will school together, and they will even school with many other types of tetra fish as well. We have covered a detailed comparison here, of the two which you might find useful.

Cardinal tetra
Image Credit: chonlasub woravichan, Shutterstock

fish divider

Final Thoughts

If you want to get yourself some lively and colorful cardinal tetras, we would recommend it for sure.

These fish are easy to care for, their tank requirements are not huge or overly specific, and they are super beautiful too!

Featured Image Credit: Kristiana Berzina, Shutterstock

Related Articles

Further Reading

Vet Articles

Latest Vet Answers

The latest veterinarians' answers to questions from our database