They are stunted, there is no two ways around that. You can easily tell by the eye to body ratio.
This is what healthy 3 1/2 month old discus look like:
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Focus on thier eyes. Imagine stacking additional eyes one on top of the other until you hit the top of the fish. See how you can easily stack 4 or more? Now look at the picture of your discus, see how you can barely stack one more eye on top? See how much larger your discus eyes are than mine in comparison to the body size? This is how you tell if the discus you are buying are juveniles, or just severly stunted fish. Yours are stunted. NO amount of clean water or good food is going to change that, the damage has already been done.
Are you changing water straight from the tap? If so you need to do a test. Test the PH of a bucket of water right out of the tap. Then fill a bucket and let it sit for 24 hours and then test the ph of that.
When/if your fish get a bit better, you need to worm them ASAP.