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Maybe a banal question, but even after 20+ years of breeding Corydoras, I haven't found a satisfactory answer. We all know that Corydoras fry are very sensitive to water quality during the first few weeks of life, but what actually causes the mass dying? Apparently, it is not ammonia, nitrities, or nitrates, because the amount of food given to them is very small. (Although some ammonia may perhaps accumulate, I have never measured it, and at pH below 7.0, it should not be harmful.)

I always thought that the fry succumbed to ectoparasitic protozoa (ectocommensals) that multiply from the waste, settle on the body of the fish, and gradually overwhelm them. In the past, I experienced multiple invasions of this sort that killed hundreds of little fry. Typically, it happened when I put them into a large tank too early and when I stopped changing water every day. The fry try to escape from the ground and are hanging on the filter. But recently, I am already not so sure if it also concerns the dying of fry during the early weeks of life in small rearing tanks. The invasions of ectocommensals are namely seasonal, wheraeas in these little tanks, little fry die constantly when water changes are not frequent. Usually, I place 10 pieces of fry per 1 liter, and still, I must change 90% water twice daily to avoid serious losses.

What shocked me most was the experience with some pieces of fry that sometimes survived in a "wastewater" tank where I was throwing infertile or moldy eggs. Obviously, from time to time, I mistook a healthy egg for a bad egg. Not only did the fry hatch in this dirty water with heaps of detritus, but they survived and lived happily! I remember that once I discovered five Corydoras napoensis swimming in the detritus, being even larger than their siblings from a rearing tank! Why didn't they die in this dirt, without any water changes?! There should be 100% mortality rate! I wrote to several ichthyologists but they weren't able to respond.

Is there some balance between microorganisms that prevents them from overgrowth? Or - as I read recently - are hatching tanks with artemia a source of harmful bacteria? But Corydoras fry die en masse even during feeding with other food...

I think that an answer to this question is very important because I am already tired of the annoying water changes twice daily. Provided that the dangerous microorganisms were neutralized, water changes could be performed once daily or even at longer intervals. I am just experimenting with acriflavine, and I am also adding a bit of fine sand into the rearing tanks (because - as someone recommended here several years ago - it protects the fry from ectocommensals).

You pose an interesting question, one with almost universal familiarity to those of us who breed corys.

I have no answers for you. Perhaps a suggestion to delay water changes is to attach a sump, 3x - 4x larger than your fry tank to filter the water.

Cheers, Eric

For marine fish there is some research saying that some fry are actually born too small to eat properly. Your experience with some fry surviving in the waste tank suggests that the mechanism may be genetic - some fry will die, some fry will survive, and if you throw a survivor in the waste tank it will survive, and even under good conditions other fry won't make it.

I don't think that it is genetic. If you don't change water, all fry die.

My experiment with acriflavine was not successful, by the way. It does not work at all. And I will probably remove sand as well because the fry cannot find food on it. Maybe, some drugs such as ESHA 2000 could be effective because I once used it with big success against infusoria liquidating my freshly hatched larvae of Corydoras duplicareus. But the drug is quite expensive and this therapy would cost much more than regular water changes. So for now, I am returning back to the classic twice-daily water change ordeal.

Some people even say that the cause of death is the bacteria layer on the bottom. Anyway, changing water and removing the layer helps a lot, and it can guarantee virtually 0% losses.

Many people end uop with good results doing opposite things. Whatever works, right? Do it.

Cheers, Eric

The incessant water changes are annoying, though. I had to move the oldest batch of fry (10 days old) to a bigger tank because of the lack of space, and two days later, the usual uncontrollable dying started. I had 32 pieces in 45 liters, and I had to change 75-80% water every 36 hours to keep them alive.

I also tried a bottom with sand, and the result was the same like in the past (disaster after disaster). This approach simply doesn't work. The uneaten food remains in the sand, it accumulates in the tank, bacteria and protozoa multiply even faster, and as a result, the fry die faster as well. Feeding with dried food is the worst. I can't even control how much food is in the tank because I don't see it. I fed them with decapsulated artemia on Sunday in the evening, and the next morning, quite expectedly, the tank was full of detritus. I quickly removed it, but on Monday in the evening, I found the first dead piece. Despite a large water change, the next day, I found another three, and on Tuesday in the morning, another one. So I had five losses within only 40 hours. I had to remove most of the sand and I left it only around the walls of the tank because they apparently like relaxing on it.

I read that a bottom with sand is primarily recommended against infections from the bacterial layer on glass, which affect barbels. So it cannot help against bacteria and protozoa in water! There are only two basic ways how to prevent the losses of fry: 1) frequent water changes, 2) elimination of bacteria and protozoa directly in the aquarium water. The second method can be accomplished using chemicals such as eSHa 2000, but it would be quite expensive in the long run and it could also harm the beneficial bacteria in the filter. So I want to test another option: the sterilization of water using a UV lamp. I just bought a small UV lamp from the Polish company Aqua El that can be attached to an internal filter. It looks like a very good idea at first glance, it is also very economical (only 1 W) and it can last up to several years.

Hi all,

cartouche wrote: ↑16 Apr 2020, 13:42 I also tried a bottom with sand, and the result was the same like in the past (disaster after disaster). This approach simply doesn't work. The uneaten food remains in the sand, it accumulates in the tank, bacteria and protozoa multiply even faster, and as a result, the fry die faster as well. Feeding with dried food is the worst. I can't even control how much food is in the tank because I don't see it. I fed them with decapsulated artemia on Sunday in the evening, and the next morning, quite expectedly, the tank was full of detritus. I quickly removed it, but on Monday in the evening, I found the first dead piece. .......I read that a bottom with sand is primarily recommended against infections from the bacterial layer on glass, which affect barbels. So it cannot help against bacteria and protozoa in water!

I'm not claiming to be a much of a fish breeder, but I've always added "Alder cones as an antifungal", "Coryman" in the thread is Ian Fuller. I'm a big moss fan as well so they get a wodge of moss and some Amazon Frogbit. All tanks are better with moss and Frogbit, because they greatly improve water quality.

Then there are snails, cherry shrimps or Asellus. They all help enormously with keeping the eggs and fry tank clean. I use either MTS or Red Ramshorn snails. Ingo Seidel uses them and that is a pretty good recommendation, we also have a thread "Live tips to save Cory eggs".

Last, but definitely not least, I like Asellus in all my tanks and Asellus are fry safe. I was actually originally told about them as a "tank janitors" for use with eggs and fry.

Have a look at Mike King's Corydoras set-ups. I just saw this the other day and I was really impressed by the set-up.



cheers Darrel

I have similar experiences when feeding protein rich food like artemia and agree it's likely to be parasitic protozoa.

What I do is use a constant dripper for auto water change to reduce maintenance schedule. On very young fry, I use a large well established tank as header tank for water source into small bare bottom 6L glass tanks or 4L ice cream containers,to hold fry.

When I feed, I tip the water level down to 2cm, flood feed with artemia then after an hour I tip the uneaten food and water out and refill.

Just add my two cents. I have kept corys for just two years and they have only spawned last year for two and a half constantly. I had more fry's that I had imagine. Literally hundreds of fry's I had to setup two extra tanks to keep them. Same as you didn't last more that 3 weeks, I was frustrated. Having hundreds of croy fry and see them die after all the care.

At the end when there were only 9 fry left I found a juvenile in the parents tank. It must have been of eggs that fell while I was collecting them to transfer to the hatching tank. Those 9 fry's were almost three weeks old so I said, what the hell, placed a sponge on the filter intake and then put them with the parents (the spawning tank). All made it to adulthood.

I think what made the difference is that this tank have been established for a year, so it had everting they needed, while the others I had to set them up because of the need for space. Even if you use filters from other tanks, plants and rocks. There is no substitute for a long term established tank.

Well that's what happened to me, what I did and what I believe is the reason. Now I have a planted tank where I keep guppies and in a moment notice I can put the guppies on the community and have a tank ready for fry's after a water change. Too bad this year there have been spawning.

I hope this help you.