STOP!!! No need to use rainwater or RO water!
If it's tank bred discus, then more than likely, it's growing in regular tap water. Discus have been bred/raised in various water parameters, including tap water. I know many discus keepers who raise and breed discus in regular tap water. Wild discus come from softer waters, but can easily adapt to regular tap water.
I have never seen peppering AND stress bars on fish. Discus that do not display stress bars have peppering and visa versa. Peppering or stress bars become more prominent with fish become stressed. Some discus display this even when not stressed. Can you post a pic of the discus?
The discus may feel scared, since it's the only discus in the tank. Plus buying already sick from the LFS doesn't really help. I have seen some discus have a change of appetite.
I would remove the discus and place it in a hospital tank, preferrably 20 or 29 gallon tank. Increase the temp, slowly, to about 86°F-90°F. Perform frequent water changes, making sure you add dechlorinator. Feed frequently, with frozen bloodworms, frozen mysis shrimp, etc. You may try to soak the food in Garlic Guard (garlic extract).