Hi. Just read thru all the new posts since I've been on.
I agree with what other posters have said.
I would up your WCs, actually to at least 50% twice a week. We like to keep nitrates below 20 ppm and lower yet is better.
Its true that a lot of Discus keepers keep adults separate from juvies due to different requirements. Juvies need more food for growout and are more susceptible to ailments than adults. With more feeding for growout, more and larger WCs are necessary.
When you start to lower your tank temp, do it slowly, a little each day. Also, if your tap is good, there's no reason to use RO. I've used my tap since I first started with Discus. I don't even age it and I deal with offgassing of CO2 by keeping my WCs to 50-60%, 2 to 3 times a week. With only a 50% WC, my PH stays perfectly stable in the tank. You can just do your WCs with tap and gradually you'll have all tap in your tank. When refilling your tank using your tap though, try to keep bubbles to a bare minimum. Too many bubbles irritate their skin and gills.
At this point, the fish with the stringy white poo should just be observed. I don't feel that medication needs to be administered at this point, either. Things may improve on their own with more frequent WCs. If it does not start to clear up in about a week or so, I'd do a Metro treatment. I've recently read in one of Untergasser's books (I think it was his) that stringy white poo, even if it is only stomach lining, is still indicative of something causing an inflamation, so I would purchase some Metro to have on hand, and there are several things that are also good to have on hand with Discus, if you don't have them already. Metro can be pricey, but
www.jehmco.com has pure Metro and sells it for a very reasonable price. There are some medicated flakes that are excellent to have as well, from
www.angelsplus.com. Dewormer flake (fenbendazole), Parasite flake (metro) and antibiotic flake (kanamycin). Also Epsom Salt, which acts as a laxative and cleans them out, regular or kosher salt for salt dips (I use Diamond Crystal Kosher with salt as the only ingredient), Quick Cure and a broad spectrum parasite med, perhaps Jungle's Parasite Clear. Honestly, I end up throwing most of the stuff out unused because it expires, but I keep this stuff on hand, just in case.
If you decide to do a salt bath, here's a link to instructions on BIDKA. They're on Simply, too. The instructions are from Paul, owner of BIDKA and also an admin on SD. He is super knowledgeable about disease/illness and treating Discus. (You might have to join to view, not sure, or you can look it up on SD. Its a sticky on both forums under Disease/Medication.
http://forum.bidka.org/showthread.php?t=161
Don't be discouraged, when you keep Discus, you're not really keeping them, you are keeping their water. They need extremely clean water, the cleaner, the better. If you can provide that and some good chow, you're all set.