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Stumped? The secrets of a Corydoras breeder by Mike Edwardes

Some Corydoras are among the easiest egglayers to breed. Such species include those which have been maintained in captivity for many generations, e.g. C.paleatus and C.anaeus. Even relatively recent arrivals such as C.panda are quite easy, at least in principle.

The basics for breeding Corydoras are:
1. healthy disease-free fish, (with both sexes present!)
2. good diet (preferably including live foods)
3. acceptable water quality

Corydoras panda
In theory, all the above conditions are simple to achieve. In practice, they are also the reason why most spawning attempts fail, and certainly the reason why the pros can often seem to achieve effortless spawnings while others struggle and fail.

Many of the Corydoras offered for sale are not healthy. This is particularly true of many (but by no means all) captive-bred specimens. As has been said so many times, find a fish store which sells healthy fish and stick to it. Even if it's a little more expensive than other stores in the area, you'll save money in the long run. Wild caught fish may also have disease problems - I once lost a hundred quids worth of wild C.barbatus due to internal parasitic infections - shh, don't tell my wife! Parastitic infections are a problem, since scaleless fish such as Corydoras are particularly sensitive to medications such as malachite green. Having said that, I routinely treat wild-caught fish with Waterlife Sterazin (a proprietary anti-parasite medication, which I believe contains malachite green and formalin). Be sure to follow the manufacturers instructions!!!

Diet and conditioning are important. Although some Corydoras will spawn on an exclusive diet of dry food, many will not, and even for the former group, the frequency of spawning and the size of the clutch will be much better when the fish are properly conditioned. Live food is also much less polluting than prepared foods, which helps in maintaining good water quality.

Corydoras paleatus
Corydoras paleatus What is acceptable water quality? Again, this depends on which species you are interested in, but I would define it as something like ammonia 0ppm, nitrate 0ppm and nitrate <30ppm. Not that I do a lot of testing - like most experienced aquarists I rely on the look and smell of the tank (no, not the taste!), plus, most importantly, the appearance and behavior of the fish. If your fish are listless and pale in colour, do a 50% waterchange and observe them for a few hours. See how their behavior changes? That's what you're aiming for.

The bacteriological quality of the water is important, it needs to be relatively clean for Corydoras (this translates as perfectly clear, no strong smell), and so is the cleanliness of the substrate. I breed all my Corydoras in tanks with bare glass bottoms. Over the years, I've tried gravel, sand, peat, etc, but bare glass consistently gives me the best results. (I paint the outsides of all my tanks (including the base) black, which helps to avoid the fish getting spooked.) I maintain my Corydoras outside of breeding attempts in tanks with a thin layer (5mm) of fine (2-3mm), smooth gravel, but this is purely cosmetic. Several German fishkeepers keep newly imported Corydoras on soft peat substrates and say important this is in helping them to settle. I've experimented with a mixture of fine pale sand (playpit sand) and peat. This looks good in my opinion and the fish seem to like it, but it does trap food and dirt and I can never maintain such good water quality with this substrate as with plain gravel or nothing, so I don't routinely use it.

There are several reason why Corydoras loose their whiskers (or barbels), another commonly reported problem:

* Gravel which is too sharp or dirty. Rub a sample between your fingers to see if there are sharp edges. Fine rather than coarse gravel is best, although some people like to keep Corydoras on soft substrates such as sand or peat rather than gravel. If you do this, only use a shallow layer.
* Bullying is another common reason why this happens. Some fish such as some barbs, cichlids or tetras nip at the whiskers, which is bad news. Choose tankmates for Corydoras carefully.
* The condition of the whiskers is a good indicator of the general condition of Corydoras. Happy, healthy fish (which you must have if you want them to breed) have long whiskers. Stress of any form (poor water conditions, bullying or disease) frequently results in loss or shortening of the whiskers.

If you fix the problem, the whiskers will regrow, but never to the original length. You often read that Corydoras with short whiskers will not breed. That's true if the fish are really stressed, but I have successfully bred Corys with short whiskers. In this case, I believe that the reason was genetic rather than environmental. Certainly in dealers tanks, I always look at the condition of the barbels as an indicator of overall health before I buy fish.

So, maybe you have all the basics right, but the fish still won't spawn - what next? Corydoras take a while to reach full sexual maturity, often several years, so patience is important. Some species are also seasonal spawners, even after several generations of captive breeding. For most species in the wild, spawning occurs in the rainy season, which corresponds to the winter period in the Northern hemisphere. This is the prime time to try for a spawning. However, if attempts to stimulate breeding fail, I usually give up and wait for 4-6 months before trying again - giving the fish longer to mature and trying at a different time of the year.

Another thing to consider is water chemistry. I am distinguishing here between water chemistry and water quality (above), and assuming the latter is OK. While most Corydoras are quite tolerant of a range of water chemistry (e.g. pH 6-8, 50-300°dH), some species, and wild-caught fish in particular are more demanding. Blackwater species such as Corydoras adolfi and its relatives (C.simulans, etc) essentially demand soft, slightly acid conditions. On the other hand, I have sometimes had success when transferring fish which have failed to spawn under these conditions back into my local tapwater (16°dH, 8°KH, pH 7.2). For Corydoras, a change is usually the key to making things happen.

Which brings us to the most important advance in fishbreeding over the last few years, the flood program. This is my standard method for inducing spawning in Corydoras. (For a good description, see Larry Vires excellent article Those Captivating Corydoras Aquarium Fish Magazine, July 1998, p30). My version goes like this:

* 8pm daily: Perform 60-80% waterchange with water a few degrees cooler than the tank. Unplug heater, turn powerhead on. Allow temperature to drift down overnight.
* 8am daily: Turn heater back on, powerhead off.
* If no response after a week or so, try 60-80% waterchanges morning AND evening.
* Important: don't give up. Larry Vires reports that it took 21 days for Corydoras virginiae to respond to this treatment.

Many Loracid species have been bred with this approach in the 1990s, including many previously considered impossible to spawn. In some cases, breeders attempt to simulate a dry season (high temperatures, low water level, aeration and feeding) for a couple of months, during which water quality is deliberately allowed to fall (don't take this to extremes!). After this, filtration with fast water flow, frequent waterchanges and heavy feeding are resumed to induce spawning.

Perhaps the most important attribute if you are serious about breeding Silurids is persistence - never give up, they'll always surprise you!