Interesting subject!
I have had the same problem here, if I translate correctly. Some of my fry develop this condition after day 6. From hatching until day 5 they will stay as normal in the cave, but after day 6 they suddenly come out of the cave and are scattered everywhere. The make a wagging movement with there bodies. The yolksack does not dissolve anymore and starts to bloat. The fry always die from this.. In my case it has nothing to do with feeding, because they don't eat at that stage. I change my water every week at least 100%. Other breeding groups don't seem to be effected and have normal fry. I also have tried everything. I can save about 25-50% of the fry if I take the eggs on day 1-2 and incubate them myself, keeping them completely separate from the rest. But it is still not the answer to the problem.... I'm interested in what other have to say about this, any tips can help
Here are some L397 fry from a year ago with the same problem:
(You don't have to proceed in English, I will try and follow as well as I can by translating your German comments
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