Blue Dimaond fry problem.

Hey guys.
I will be very appreciate for any information about my problem.
Approx. after 2 weeks old blue diamond discus fry stop growing(they eating) and they darken(almost blacks).
Some of them has tail rot, some - swimming together on the top of the fish tanks.
It goes in the next 2-3 weeks, but in this period 10%-15% of batch for no reason turning to light blue and they begin to grow, as it should be if fry is healthy
During that time, some of them dying(not a lot) but anyway, they all will dead except those, who look healthy.
Any suggestions?
Metronidazole,Nitrofurazon, Furazolidon doesn't help
I have many strains to breed and only with blue diamonds fry I have a problem

Thank you.
Mark