Thank you all for your answers!
To Helene: the water color changes from time to time a bit. And I had much light on to make the photos. I use Deepin Peats black water recipe. If I have suck the elder cones too long, the water is so dark I nearly don’t see anything, and impossible to make good photos.
To Peter: I observe the fish since about two weeks. No changes.
To Bill: Thank you for your very good photo! No, this way no one of the fish look like. I had some time ago fish which looked like that, I wrote about it in the forum (phoenicurus) and I was able to save/heal them all, even in a late stage with spirohexol JBL in the liquid form (I couldn’t get here the powder). And I used it in the awareness that it was written, it would not help. But it did.