Hi Raul,
Andrew Soh has written two very helpful books on discus, and as far as I'm aware the first one should be available in Spanish.
In one of his books he recommends a treatment for fry that have gone black and the treatment is a PP bath. This is why I asked if you have used it before or not as sometimes the dose level to treat the fish is close to killing them so it is not something that I would recommend for beginners..................JMO :wink;
If you are happy using PP then I have found two articles that are invaluable when it comes to bathing in PP.
Have a read of these and let me know if you think they will work. if you have any concerns then let me know

HTH
Best Regards......................Tony