I carry out a 25% waterchange about 2 times a week with r/o water yet one moment my discus are fine next they turn black off and on all the time.
Do you check the in coming of 25% water into your tank?
Whether they contain Chlorine?
Do you apply Chlorine conditioner? (Hypo)
Apply antibiotic plus salt (Oxtetracyline or tetracycline).
Water changed and repeat the treatment for a week.