Nu2plants already mentioned most of the following, of which you should take note.
I'll repeat for emphasis, by giving my take on your situation:
You have very young fish, barely into the juvenile stage, whose immune systems are not yet fully developed. As such, they are susceptible to any number of pathogens, and require certain conditions while growing out, which they do not have the benefit of at the moment in your planted tank with substrate.
Firstly though, it would be of interest to know the source of your discus - did you obtain them from a breeder or importer of quality fish ? In circumstances such as yours, one has to wonder whether the fish were healthy when you got them.
Assuming they were, and eating at the time, the conditions you're keeping those youngsters in are far from ideal for successfully growing them out, and it's not surprising they are showing signs of ill health.
What I strongly suggest you need to do now, and maintain at least over the next few months until they mature more, is to remove all of the substrate/plants/& other décor from the tank, and keep them 'bare-bottom'.
Undertake for the next month or more to do large, daily water changes - 70% or more, along with vacuuming of wastes, and tank wall wipe-downs. Feed your fish small amounts, several times a day - up to 5 or 6, with good amounts of protein, e.g. freeze-dried blackworms, a beef heart mix, brine & Mysis shrimp, as examples. (Bloodworms are not very nutritious and should be fed only in moderation).
Try the mix-in with fresh minced garlic as you already mentioned, if you need to entice them to take new foods - AND raise the temp to at least 30 C or 31C.
That way, you'll be giving those young fish the best chances to develop properly.