You mentioned you use chemiclean to combat cyano right? I believe should be this reason, If you never dose correctly this will cause your oxygen and parameter to swing. You need to on your skimmer during the treatment but removing the skimmer cup so that you tank got enough oxygen. Also to monitor ammonia every 2 hrs to check for spike. If you fish is still eating healthy, go buy seachem metroplex & focus and add in their food to help combat the ick. If not Dr's G Anti-parasite food also can. Catch the fish to QT now may stress them further, maybe after they recover a bit then u can move to QT and leave ur main tank fishless for 3 mths. Gd luck!