Welcome to SD. I see you've been waiting most of the day for replies. So sorry your beautiful fish are not well.
I assume that some of the more knowledgeable members have not replied because it is a planted tank. Many diseases are hard to impossible to treat and eradicate in a planted tank. I am not a biologist, but can make a few observations:
1) The rocks in the tank look like they came out of a salt water tank. That looks like possibly cyanobacteria on the rocks. Something growing on or emitting from the rocks could be making the fish sick. Removing the rocks would be a first step.
2) Discus are easily startled and intimidated, even by smaller fish. In my opinion you have too many non-discus in the tank with them. Discus sleep at night, often resting on the bottom. All those bottom feeders moving around at night could be stressing the discus.
3) Water changes. People often think they can do less water changes in a planted tank because the plants consume the waste. The reality is that planted tanks require as much or more water change as bare bottom. Plants and substrate help create a climate where harmful bacteria and parasites can multiply, especially with small infrequent water changes.
4) White feces is a sign that something is not right, but does not always call for medication. You have not said what you are feeding. A change of diet may be in order. A variety of foods including high protein, vegetation, and vitamin enrichment is best.
5) Most on SD do not recommend using pure RO water. What is the purpose of that? You are missing certain minerals and trace elements using 100% RO water. Most use tap water unless breeding. Aged if they have a big ph swing.
I started with a gravel bottom mixed species tank. After fighting disease for weeks, I removed the substrate and non-discus fish. No sickness issues since. If you do remove the substrate, do it carefully.
My opinion. May not be everyone's opinion. Those green discus are nice. I would rehome all of the non-discus, put the discus in a 5 gallon bucket of tank water, drain the tank and remove the substrate. Refill with warm dechlorinated tap water and put the discus back in. Now you have room for a new group of discus. You could also downsize to a smaller tank. Again, this is one opinion. I believe the discus would be more healthy and easier to keep healthy. Others who successfully keep planted tanks might want to add other options.
Best Wishes