Thank’s for your answers, Helene and Bill.
Bill – I can’t say it for sure, but I don’t have the impression that there is something wrong with the fry or that she picks out only certain young. I see her “scanning” the hanging roots of the plants floating on the surface where the small fry like to hide. To me she is simply hunting – it doesn’t look different to hunting glass worms.
And as I said – the small fry are very secretive and obviously well aware of their dangerous mother. As they grow (too big to be swallowed) they lose their fear and swim freely right in front of their mother (which obviously checks whether they are still “edible” or not).