Have you had these discus long, or did you just get them. They look dark and very badly stunted. Look at their eyes. Very big for the size of fish.
I couldn't get the video to work, I'm getting an error. But from the pics, is that all fish poop in the sand? Discus won't be able to deal with all that gunk. They demand pristine water and tank. Are you feeding them about 6 times a day? They look a bit thin, as well as stunted. Is the temp at least 82f or 28c?
They won't tolerate nitrates. If you have over about 5 ppm nitrate, you need to lower those quickly. Twice a week water changes might not be enough. Also don't bother with piddly little 10-20% changes. They want huge water changes, 90% each time is more their speed. As many times per week as needed to keep nitrates below 5 ppm.
When you test for nitrates, shake the bottle at least for 3 minutes before using them. The reagents precipitate out of solution. You need to shake very vigorously for at least three minutes to re- suspend it. Otherwise you get false low nitrate numbers.