Cory catfish are among the most popular bottom dwellers in home aquariums. However, when I first thought of owning some, I had no idea how many to get.
Over the years, I have learned that there are many variables to this question, including the specific type of cory, the size of the tank, additional fish in the tank, etc.
In this article, I will help you decide how many corydoras you need to get in your particular tank setup. Let’s dive right into it.
How Many Cory Catfish Should Be Kept Together?
The ideal number of cory catfish in a tank is six. You should keep them in a tank of about 20 gallons, without any specific ratio between male and female.
Of course, not all cory catfish are the same. There are different subtypes, each with its own size and characteristics.
Not every tank is the same either, as yours has its own unique size, shape, and fish population.
While a group of six in a 20-gallon tank is the general recommendation, this may not be the case in your particular tank.
That is why I suggest following these three steps:
1. Find The Average Size Of Your Cory Catfish
The size complicates matters where most other species are concerned because it changes depending on the type.
But cory catfish are small creatures. Most cory catfish types fall within the 1 to 4 inches size range.
Pygmy corydoras are the smallest at 0.75 inches, followed closely by dwarf corydoras at 1.4 inches and salt and pepper corydoras at 1.4 inches.
But what if a retailer tricks you into buying one of the larger cory catfish? What happens, then?
The biggest fish are the banded cory catfish (4 inches), emerald cory catfish (3.5 inches), and peppered cory catfish (3 inches).[1]
Some people have tried to classify the Brochis fish as some sort of giant corydoras catfish.
Dr. I.J.H. Isbrucker explored its place within the Corydoradinae subfamily in the Catfish Association Great Britain Magazine.[2]
But even if you accept that classification, the Brochis grows to 3.5 inches in length, which is not particularly impressive.[3]
The other cory catfish types fall somewhere in between, including:
- Bandit Cory – 2.5 inches
- Bronze Cory – 2.5 inches
- Three Stripe Cory – 2.5 inches
- Skunk Cory – 2 inches
- Julii Cory – 2.5 inches
- Panda Cory – 2 inches
- Albino Cory – 2.75 inches
- Habrosus Cory – 1.5 inches
- Paleatus Cory – 2.5 inches
- Sterba’s Cory – 2.5 inches
- Similis Cory – 2 inches
- Adolfo Cory – 2.5 inches
- False Julii Cory – 2 inches
Simply put, keeping multiple cory catfish in the same tank isn’t difficult because of their small size. Even in large groups, you don’t expect the species to crowd a sizable tank.
2. Adjust The Number Of Corys To Your Tank Size
Keeping one cory catfish is easy. The smallest of their kind may even survive in a bowl. But the same cannot be said for a group.
What tank sizes do you have on hand?
- 5-Gallon Tanks
Five gallons are out of the question. Have you heard of the ‘One Inch Of Fish Per Gallon’ rule? You need one gallon of water for every inch of fish.
In other words, a 4-inch cory catfish needs 4 gallons of water. Two 4-inch cory catfish would die in a 5-gallon tank.
- 10-Gallon Tanks
People often tout ten gallons as the minimum tank size for a cory catfish, but that is a mistake.
Ten gallons can accommodate a conventional school of six catfish. But you need the smallest types for the group to thrive.
That includes Pygmy, dwarf, and pepper corydoras. Either way, don’t exceed six fish. Otherwise, ten gallons are not enough.
- 15-Gallon Tanks
Any school of six cory catfish that survives a 10-gallon tank will also tolerate 15 gallons.
Again, you should stick to smaller cory catfish, preferably ones that grow up to two inches. Reduce the number of fish if you prefer larger 4-inch cory catfish.
- 20-Gallon Tanks
Twenty gallons is the optimal size for a cory catfish tank. It can accommodate every cory catfish type on the market in a group of six.
Although, you should limit your selection to the smallest cory catfish for groups larger than eight individuals.
Many aquarists prefer a 20-gallon tank because it opens the door for other species to live alongside their corydoras, including guppies, loaches, plecos, neon tetras, shrimp, and even snails.
- 29/30-Gallon Tanks
Thirty gallons is an excellent size for a cory catfish tank. If all you grow are corydoras, it can easily accommodate a group of twelve.
Obviously, if your tank also holds other fish and has plenty of plants and decorations, twelve may be too many.
In this case, you can start by introducing 6-8 Cory catfish. If you want to add more, do it gradually and see how your fish react.
Remember that it is always a good idea to leave extra room for cory catfish, as they multiply quite quickly. During the breeding season, they can lay eggs every seven days.
3. Don’t Rely Solely On The ‘One Inch Per Gallon Rule’
This rule is complicated because you can’t apply it directly. There are a few things to keep in mind.
First, fish are not the same size. For instance, pygmy corydoras have an average length of 0.75 inches.
But even if you decided to populate your tank with pygmy corydoras and nothing else, you can’t expect every pygmy cory to grow to the same size.
Some of them may exceed 0.75 inches significantly. Others may fall below that average size.
Second, this rule only considers the length of the fish. It ignores the width, which is a mistake because fish get wider as they mature.
Lastly, fish are not the only objects that occupy a tank. You have plants, rocks, decorations, and even the substrate.
Therefore, a 10-gallon tank has less than 10 gallons of water to offer (8.5 gallons) by the time you introduce the fish.[4]
Do Cory Catfish Have To Be In Groups?
You cannot keep cory catfish alone in an aquarium because they are shoaling fish.
A study in the Journal of Experimental Biology investigated the impact of shoaling habits on the metabolic rate in fish.[5]
First of all, they found that shoaling had a calming effect on social animals. Secondly, their results proved that isolation among shoaling fish had negative psychological consequences.
The stress that isolation causes in cory catfish could kill them because they are happiest in groups. But how many neighbors do they need?
- Can You Keep Two Cory Catfish?
If you can’t keep one cory catfish, a unit of two seems acceptable.
However, a team from the Universities of Plymouth and Exeter researched the matter and concluded that a group of two fish was too small to satisfy the demands of shoaling species.[6]
Two cory catfish are better than one. But the fish won’t feel secure until you increase their numbers.
- Can You Keep Three Cory Catfish?
Yes, you can, but you are treading in dangerous waters. Cory catfish in a group of 3 can still succumb to stress.
Shoaling makes these creatures feel secure. They are less likely to fall prey to predators. Therefore, they are happiest in larger groups.
- Can You Keep Four Cory Catfish?
If you can afford to raise three cory catfish, you might as well add a fourth one. A group of four can work. Anything less is unacceptable.
- Can You Keep Five Cory Catfish?
Six is the best number. But if you don’t have room for six cory catfish, five will do.
Can I Mix Different Types Of Cory Catfish?
According to a paper published by Jennifer L Snekser, Nathan Ruhl, Kristoffer Bauer, and Scott P. McRobert, it makes more sense to buy the same type of cory catfish.
They found that shoals of fish with similar phenotypic characteristics were safer because predators had a difficult time identifying specific individuals to attack.[7]
However, the paper also admitted that fish were more than happy to join shoals with different types of fish. They did not discriminate while choosing shoal mates.
Therefore, you have no reason to believe that different types of cory catfish will reject one another.
Does The Male To Female Ratio Matter?
This is a valid question because many types of fish require a certain balance between males and females to avoid aggression.
For example, platies need three females for every male. You have a similar 3:1 ratio among guppies as well.
But cory catfish are too peaceful to harass one another. You don’t have to provide a specific number of female fish to keep the male cory catfish happy.
The ratio between male and female simply does not matter among this type of fish.
Conclusions
As a rule of thumb, it is best to grow a group of six corydoras in a 20-gallon tank. If you have a smaller tank, it would be best to pick relatively small species.
That may include pygmy corydoras, dwarf corydoras, and salt and pepper corydoras, which do not exceed 2 inches.
You can keep cory catfish in smaller groups of five or even four individuals. However, Corydoras are shoaling fish and do best in a group of six or more.
References
- https://aquariumfishcity.com/how-big-do-cory-catfish-get/
- https://www.researchgate.net/publication/311741695_Brochis_britskii_a_rare_plated_catfish_from_Brazil
- https://www.practicalfishkeeping.co.uk/features/brochis-a-giant-among-dwarfs/
- https://www.fishtankworld.com/one-inch-of-fish-per-gallon-rule/
- https://journals.biologists.com/jeb/article/219/18/2802/15433/Shoaling-reduces-metabolic-rate-in-a-gregarious
- https://phys.org/news/2009-06-company-fish.html
- https://escholarship.org/content/qt8n68z3pf/qt8n68z3pf_noSplash_55cb0d7cfdcad3ac904d5cab5f2c3ab1.pdf?t=mwj0qb