The Corydoras group of catfish are well known amongst fish keepers and are an excellent fish for the community aquarium. These are peaceful, relatively robust fish with loads of character, and there are many varieties to choose from.
Providing the water is well filtered and regularly changed, and the fish are kept in groups, any fish keeper should be able to keep these little scavenging fish successfully.
Corydoras are from the Callichthyidae family of armoured catfishes, having a hard, armoured body. The name 'Corydoras' actually comes from the Greek for helmet (kory) and skin (doras)
The Corydoras genus contains over 180 species, with hundreds more unclassified specimens which are assigned 'C numbers'. In addition to this, there are also variants of many species, which are sometimes a matter of debate as to whether they are different species, or just geographical variations. Corydoras range in size from 2.5cm (1 Inch) to 12cm (5 Inches), with most reaching between 4-8cm.
Whilst various species crop up for sale regularly, it is usually fairly common specimens from about 20-30 different species which are seen for sale in the trade. Beware of any very bright colours on Corydoras, since they have in the past been a victim of the cruel practice of 'dying' by injection or other method.
Corydoras come from South America, between Argentine and Columbia, with the vast majority originating from the Amazon basin. Their natural habitat is in clear, slow moving streams and shallow rivers, usually where there is vegetation at the banks. Although they can be found in gravely areas, they prefer a softer substrate and are usually seen in sandy or muddy-bottomed areas, where there is plenty of leaf litter or mulm.
Corydoras will scavenge amongst the detritus, where plenty of aquatic organisms can be found, and retreat to the safety of the vegetated riverbank when required. These types of habitat are often collection areas for larger rivers and are affected seasonally by the rains, which cause a drop in water temperature, and are a trigger for spawning. Because of this, many Corydoras can be kept in cooler water down to 18-20C (64-68F), but not all, so check before keeping them in unheated tanks.
Corydoras are a peaceful and versatile fish; they never squabble and will not harass other tank mates. In the aquarium Corydoras seem to have two modes, and they will be either resting on the substrate, or actively searching for food.
It is important to keep these fish in groups, and they will stick together, especially when resting, although they will often only group with other Corydoras of the same species. An unusual behaviour can be seen when the fish dart to the surface, take a gulp of air, and head back down to the substrate. This is perfectly normal - Corydoras have the ability to absorb oxygen from atmospheric air, and it is a normal part of their respiration.
Recreating the types of microhabitat the fish have in nature is very important for the overall health of the fish, and will encourage more active and interesting behaviour. The aquarium should have three distinct areas of hiding spots, vegetation, and open areas.
Hiding spots can be made from pieces of bogwood, or any decor that creates a subdued and enclosed area where the Corydoras can sit without being harassed by other fish. Vegetation is important and the best types are bushy plants with lots of stems at the base. Cryptocoryne sp. and carpeting plants are ideal, and the Corydoras will enjoy rummaging amongst the base to find scraps of trapped food.
Open areas are also used for scavenging activities, and the most important thing here is the substrate, which should be fine rounded gravel or sand. Sharp gravel will damage the fish's sensitive barbells and mouthparts, and larger gravel is too heavy and bulky to dig into.
Corydoras are relatively easy to mix with other fish, and will do well in most community aquariums. Because of their armour plating, they can often be kept with fish up to four times their size without problem, although aggressive or heavily territorial fish are best avoided.
Corydoras do not bother other fish, so small or delicate species will also make good tank mates. Although Corydoras are not large fish, they do like a bit of swimming space so a 60cm (two foot) tank should be a minimum size. Since they like to rest occasionally, it is best to avoid any over-active bottom dwellers, which may disturb or stress the Corydoras.
Whilst Corydoras are adaptable in relation to water temperature and fluctuating conditions, they are very sensitive to chemicals, toxins, and pollutants. Nitrates in particular should be carefully monitored in aquariums with Corydoras, since they do not cope well with high levels, which often result in infection of the barbells.
It is wise to use some carbon, or other chemical filtration media at least occasionally, since airborne pollutants can cause health difficulties. Some strong treatments will also cause problems, so always be careful which chemicals you use in the aquarium.
Plenty of regular water changes, with conditioned water will ensure good long-term conditions. In terms of pH and hardness, most Corydoras prefer slightly soft, acidic water, although many of the common species can be kept in harder water quite happily
In the wild Corydoras feed on a variety of bottom dwelling insects, aquatic organisms, larvae, and worms by scavenging amongst the substrate, often digging their snouts in to find food items. This can be recreated in the aquarium by feeding a range of live or frozen foods. Whilst other fish in the aquarium will get the bulk of these foods as they sink down, the bits which get missed will be quickly found by the Corydoras over the next hour or so.
Living off scraps however is not good enough so make sure you feed regular sinking pellet or wafer foods, which also contain important vitamins and minerals. Corydoras will eat flake food, although if you are feeding flake and significant amounts are reaching the bottom, you are probably overfeeding the tank.
Some Corydoras will breed easier than others, but in general most can be bred in the aquarium. Attempting to duplicate the natural water conditions of the species being kept will help a great deal, but this may require some further research. Corydoras can often be encouraged to spawn by a change in water conditions, mimicking the natural trigger of the rainy season. To do this, it is best to use distilled or reverse osmosis water, and carry out a series of significant water changes at the same time as dropping the water temperature by a few degrees.
If you want to be really accurate, you can even wait until a cold spell or storm - the drop in air pressure will contribute to the trigger effect. The use of bogwood, which releases natural tannins, can also help. Corydoras have various breeding habits, so it is best to look up the particular species you are breeding, and be ready to remove eggs or young to a separate rearing tank or area away from other fish.
Whilst there are hundreds of Corydoras species, a few are regular characters in aquarium retailers, and these are the most likely candidates for a fish keepers first foray into Corydoras keeping.
The most common of all is the Bronze Corydoras C. aeneus, which is the same species as the almost equally common Albino Corydoras. Along with the Peppered Corydoras, C. paleatus, these three fish are some of the hardiest and easiest to keep Corydoras and can be kept in unheated aquariums (but not with goldfish, which will get to large and boisterous)
Smaller popular Corydoras include the intricately marked C. Julii and the distinctively markedPanda Corydoras C. panda, both of which are beautiful little fish, ideal for smaller communities or planted aquariums. There are also a few really small Corydoras, which include the Salt & Pepper Corydoras C. habrosus, and the Pygmy Corydoras C. pygmaeus which grow to only around 2-3cm. Whilst popular, and a good addition to small communities, tiny Corydoras like these are a lot more delicate and will need regular feeds on very small foods such as Cyclops or Daphnia.
For something a big bigger, Corydoras Sterbai is a very nice looking fish, with intricate body markings and an orange edging on the pectoral fins. Beware of transporting Corydoras species with colour on the pectoral fins, since this is often a sign that the fish are one of the few Corydoras that will release a chemical toxin when stressed, and they can kill themselves during transport if they are overcrowded or over stressed.
Whichever Corydoras you choose to keep, they may well become a favoured group in the aquarium, and you will probably keep them for as long as you keep tropical community fish.