The Cory Tank
Suitable Sizes, Lengths, and Setup
There are a few things you need to consider when you setup a tank where you want to keep Corydoras, Aspidoras, Brochis or Scleromystax.
Please note that Corydoras are not "the cleaning crew" of your tank. They need to be fed proper food tailored for them or they will never thrive.
Tank size
The size of the tank depends on how large of a group you want to keep. Normally you should keep 6-9 animals since they are schooling fish. They don't necessarily have to be the same species, you could have 3 each of 3 species, but note that they prefer schooling with their own species.
There are two exceptions, though:
1: Dwarf species (Corydoras hastatus, Corydoras habrosus, Aspidoras pauciradiatus, etc.) should be kept in groups of about 10 (or more). Some of them swim in mid water and might be shy if they are too few.
2: Many long nosed Corydoras species and Scleromystax are VERY territorial (not the cute lite fish we associate with Corydoras). Keep 2 males and 3-4 females in a large enough tank is recommended.
To keep a group of 6-9 animals follow these suggestions.
- The really large species like Corydoras semiaquilus, Corydoras narcissus, Brochis multiradiatus and B. britski need a tank of about 200 l (50 gal). Even that might be too small since some of the large Corydoras and some of the long nosed species are VERY territorial and will kill the other fish of the same species!
- Large species like Scleromystax barbatus need a tank of more than a 100 l (25 gal). Long-snouted males can be very territorial and the largest male might kill the smaller ones! I only had a male C. cervinus left after he had killed 3 females. Even the fry can be territorial from a very young age.
Who said that all Corydoras are nice and friendly? - The slightly smaller like Corydoras geryi, Corydoras virginiae and Corydoras sp. C005 between 75 and 100 l (20-25 gal).
- The bulk of the species will do nicely in 30-50 l (8-13 gal).
- The dwarf species like Corydoras hastatus, Corydoras habrosus, and Corydoras pygmaeus can be housed in 20 l (5 gal).
- In the same tank you can of course keep other fish. Remember never to overstock though!
Tank sizes for breeding purposes see Breeding Corydoras.
Filtration
Some of the more sensitive species (i.e. Corydoras nijsseni and C. concolor) need a very good filtration. Otherwise they are not that fuzzy.
For small tanks less than 50 l (13 gal) a LARGE sponge filter (larger than fist-size) with good air flow can be used. Remember to clean it regularly (every two to three months, depending on fish load).
For larger tanks an internal power filter or an external canister filter is among the best.
Water
I have a lot to write about water, RO water, chlorine, chloramines, and copper in the tap water, but will get back to that later on.
Lighting
Not critical but if you keep plants in the tank you should adapt the light level for the plants. Some species are easily scared if the tank is to well lit and will lurk in the shadows. On the other hand, a too dark tank in a well lit room can also be bad since you will cast a shadow on the tank and therefore scare the fish.
A species that huddles under some roots almost never to be seen in one tank, will be laying out in the open at the front glass in an other. I just don't understand why, maybe the lighting in the room and room background?
Substrate
Whether to use bottom material or not can be debated. The most common is to have some kind of gravel or sand. When a bottom material is used it will help increase the surface for good bacteria to multiply.
A problem with Corydoras is eroding barbels. Some people say that having sharp gravel can erode the barbels on Corydoras. I DON'T believe in that. Where we collected Corydoras in Uruguay there were lots of sharp stones and rocks with lots of Corys feeding among them. Some were over fine sand as well so...
Of course glass or something similarly sharp would be bad!
I have identical gravel and sand in all my tank. Some have pie size gravel and the other have the finest beach sand you can find. In both these tanks I can see barbel erosion on the more sensitive species if the water quality is bad (meaning I don't change enough water and the filter is not cleaned often enough). Some of the more sensitive species are C. concolor, habrosus and wild caught longipinnis (at least those
from Uruguay).
If you look at a healthy Corydoras working it's way through the gravel you see that the barbels are not sensitive to wear. They move rather large gravel without a problem.
The problem is probably bacterial build-up in the gravel if you don't do your maintenance.
Advantages with bottom material
- Some species prefer a dark bottom, other a pale one. Some pale Corydoras show a better contrast on a pale bottom. You could also paint the bottom.
- Many (all?) species "like" to probe around in the bottom for food. The long nosed species really seem to be built to do that.
- Less risk of fungus attack on fry
- No reflections from the bottom (painting the outside black will reduce the reflection)
Disadvantages with bottom material
- Difficult to see if all food has been eaten
- Difficult to clean without vacuuming out sand
Heating
Keep the temperature at 22-25°C (72-78°F). If you need a heater depends on the ambient temperature and the heat generated from the lighting and your power head (it works as a heater that is always turned on).
Some species can be kept at higher temperatures (i.e. Corydoras sterbai, C. concolor, and C. julii) at 28-29°C (82-84°F), while some of the highland or southern species (i.e. Scleromystax barbatus, S. macropterus, Cordyroas undualtus, C. longipinnis) must usually be held at temperatures lower than 22°C (72°F) to survive for a longer period in the tank.
I've had a pair of S. barbatus that survived the Swedish summer of 2002 without any problem. It was rarely below 26°C (79°F) for months! Keep them in the lower tanks if you have several to choose from.
The C. longipinnis I've collected in Uruguay in 1997 and some C. undulatus I've bred survive nicely in my largest tank with 28-29°C (82-84°F) for a couple of months even though they come from the cooler parts of South America!