I only speak from my experiences, when I had two oscars one tiger one albino a pleco and a jd, when I added the jd and didn't change things up a bit my oscars hid in a corner and wouldn't come out for anything. No food not to play nothing! Then I changed the decor by chance and it worked. My albino was the tank boss and when the jd came in he took over scaring the crap out of my other two. I only suggest it because it worked for me. You said your parameters are all good that there's no sign of infection, try switching up your tank a little and see what happens?