WHOA, slow down! ha ha. One step at a time... You're making my head spin a little.
The good news is you are WAY devoted to your water changes so you will certainly be a successful discus keeper.
You'll get there, but you need to stop making snap decisions and incorrect diagnosis, you're wasting you time and money. Remember nothing makes a discus happier than pure warm H20 with nothing else in it- so don't jump to add anything- slow and steady...
Couple things- always deworm new discus to be safe. Feed them a flake (or soak bloodworms) with fenbendazole for a week or so.
Quit moving fish around and don't ever add anyone to your discus tank that hasn't been quarantined for at least 30 days AND dewormed. That means by itself and not in and out of other tanks with other fish. That is the ABSOLUTE WORST thing you can do. Granted I have done it- (BAD PAULA! BAD!) but when I do- everyone in the tank eats Strictly medicated flake for at least two weeks and its fish from breeders I know- never imports- but don't do it. Do as I say, not as I do ;-P. Its really playing with fire.
You said you were treating ich. Fish can not get ich at 87 degrees. Ich can not attach to fish in temps over 82. Check your heat. If you are at 87, you didn't get ich, the meds, the salt, all a waste and a stress on your fish. Don't go near the tank with rid ich again. Ever. Seriously- I could drop a fish loaded with ich in my tanks and not worry at all- its very difficult for ich to get a foot hold on healthy fish and its impossible at 87 degrees. Impossible. I think i might actually do it just to post the photos and prove a point. It is possible the fish came with a spot or two of ich, but if it was not a severe outbreak and the fish wasn't COVERED in it- don't sweat it. Let it live out its week long life cycle and its gone....
Take it easy, slow and steady, and don't add anything- meds, tank mates- ANYTHING, without careful deliberation and tons of research from good scientific sources.
If you have planaria (little scooting mites and white worms on the glass) they will NOT attack or effect your discus. perfectly harmless and totally normal. The prazi pro will probably kill them anyway.
Also, planaria/nematodes in a tank with daily 30% w/c is extremely uncommon- unless you are not gravel vac-ing out uneaten food, or if the food is getting sucked up and caught in your filter before the fish get a chance to eat it. They tend to feast and multiply on detritus and left over fish food in the tank. They are totally harmless. Common sights are hairlike white worms less than 1/2" in length wiggling in the water column, short fatter flat white worms slowly crawling on glass and gravel with arrow shaped heads, and little zippy mites that kind of shake and sputter in the water. These are all harmless, but an indication that somewhere in the tank is some rotting organic matter- like extra fish food. Make sure when you W/C you gravel vac the bottom of the tank to get up uneaten food.
Turn your filter back on right away. Don't ever turn your filter off. Ever. Your bacteria colony needs O2 from flowing water to survive. If you cut off that oxygen supply to the bacteria for any length of time you risk destroying your biofilter and could have to re-cycle your tank.
What you might want to do is - on your next water change use the old tank water, and rinse all your filter media in it really well (never use tap water to clean filter media) If you have a build up of food caught in your filter material that could explain why the nematodes are coming out of your filter return.
You were right to stop feeling live black worms. I know the fish love them, but they are really not clean pathologically speaking, and it just isn't worth the risk.
Forget about looking for a large white worm coming out of your discus - that's nonsense. Just deworm them well. If you've already bought it, use the parazipro. Its very gentle and pretty effective. Also, jungle parasite tabs work great- a mix of prazaquintel, metronidazole and another I can't remember- flubendazole maybe? Those may all be spelled wrong, don't hold it against me.
If you still notice white stringy feces after treatment, give them with food soaked in fenbendazole or better, a flake that's already been prepared with it- the fish aren't going to want to eat it at first, be remember they are that way anytime you start on a new food. You can't add it to the water like prazipro or jungle tabs, but it is a VERY effective deworming treatment.
Where did you get the Discus? Were they raised in a local hatchery or imported?
Take your time, do your water changes, and let us know how it goes.