I for one do not think you have discus plague.
Don't complicate or worsen matters by undertaking the medicating that you now say you want to do - that's bad news at this stage, imo.
Your discus simply appear to be in very poor condition, - unhealthy to some extent, probably due to being grown out/raised under less than adequate tank conditions and in poor water quality.
Their fins are ragged and need healing, and should be treated with salt.
No medications of any kind yet - that could only worsen matters.
You need to remedy the main problem as I see it, and that is easily done with some effort on your part - from what I can see, the sides and back of your tank are dirty, and from the overall look of things in the tank (which includes decomposing plant matter), I do not think they have been receiving sufficient tank cleansing care & appropriate, sufficiently large and frequent water changes.
As I mentioned before, if you want to bring those fish back to good health and save them, I strongly urge you to do the following:
Do a large 50% to 60% wc, and at the same time, give the tank a complete wipe-down of all glass walls (don't use that magnetic algae srcubber, use a good sponge or paper towelling). Also do a very significant vacuuming of your substrate, to clean it up as best you can. Refill with conditioned water & give those fish a half portion recommended salt bath dosage.
Repeat this routine every second or third day for the next 2 weeks or so to really clean up that tank. Then maintain a minimum twice weekly regime of tank clean-up & wcs to maintain good water quality & conditions.
For the time being, forget about your CO2 dosing - stop it until you get those discus back to good health. They don't need any pH swings, or other complications, just good, clean, fresh water & a clean tank.
Over the next few days, clean up your filtration unit(s), rinse the media well in siphoned out tank water (with a wc), replacing any media that seems beyond salvation.
You would be very surprised how much better those discus can look (and regain some appetite) with lots of fresh clean water, and sufficient care and attention to tank cleanliness.
Give it a shot - it will work !