Hi and welcome aboard Subek .
Nice tank you got there , how big is it ?
Your soft and acidic tap water is perfect for raising and even breeding discus . Are those Warsaw tap parameters?
You have made a mistake introducing and mixing discus on multiple occasions without even quarantining them . Maybe the problems with your last 2 discus that you experiencing right now are due to cross contamination .
Other and most important thing to consider for discus health is the water quality . Your deep and coarse sand bed is notorious for trapping dirt and compromising the water quality .Its dark colour also make your discus look darker and your pigeon blood discus peppered . I would consider Changing to 1 cm Pool filter sand if I were you.
How often and how much you change water , or siphon the sand bed ?
What kind of filtration and flow do you have ?
You got baby discus with weak immune systems that are very intolerant of anything but perfect water quality . So you'll have to work real hard on that first .
Rule out the water quality issues (if any) and if after 2-3weeks your discus still won't eat than you can put them in a quarantine tank and start with elevated temperature treatment or consider some other sort of treatments .
Cheers and good luck .