Bronze Corydoras fry dying after 6 weeks

[willemp]

I was lucky to buy a male and female cory. They have been spawning since I got them.

I had a fungus problem with the cory fry in the breeding tank and they all died.

I then noticed about 5 fry in the main tank (the eggs were not eaten). They were approximately 2 weeks old when I first saw them.

I just left them and they grew up nicely to about 1.5 to 1.8 cm.

Since week 6 they started dying. One after the other they disappeared. I saw one of the last two just lying on his side but not dead. I “buried” the fry and about two days later I saw the last one doing the same. He seemed healthy beforehand.

He was lying on his side at times, and it was not difficult catching him. I put him in the breeding tank and added some Methylene Blue (out of desperation as I really wanted to save him). I also got some API Furan-2 Anti-Bacterial medication.

That was 2 weeks ago and he is still alive, but he still behaves the same - sometimes lying on his side as if to die, but then swimming off, bumping into the glass, etc.

I do daily water changes and feed him the same the other catfish in the main tank receive – Nutrafin Max Bottom Feeder Sinking Food Tablets and TetraVeggie Algae Wafers.

The water readings in the main tank are:

Ammonia – 0

Nitrite – 0

Nitrate – 10

PH – 7.0

KH – 2

Temperature 26 C

I do weekly changes of 30 to 40L. The tank size is 140L and I have an Eheim Professional filter,

My main question is:

Why, if the fry grew up in the main tank for 6 to 7 weeks, do they start dying only then? I do appreciate that there is a big mortality rate. I would have expected them all to die very young because they did not get the right food etc, but they were getting nice and big and eating with the adults and other fish.

Moving the eggs to the breading tank doesn’t help as they start getting fungus after a few weeks (the last batch got fungus after 4 weeks) and even putting Methyl Blue in after I saw it, didn’t help.

The other fish in the tank are:

Zebra Danio

Rasboras

Rainbow Shark

Bristlenose catfish

Bronze Cory

None of the fish are aggressive and never bothered the fry.

I have lots of plants and hiding places for them.

I will appreciate any help on why they die at such an advanced age of development. I just doesn’t make sense to me.

Thanks