Hello Christian,
If all else fails you can always try soaking the food in Garlic Guard by Seachem if your discus wont eat or are fussy eaters, you can order it online from ebay. What are you feeding them? Are the Discus the only fish in the tank? I imagine your tank was cycled before you put them in it? What is the temp of the water? Do you have test kit readings you could share with us of your water quality?