Older peacocks dying off

I have an issue with a mystery illness that is killing off my oldest peacock cichlids. I also have Brichardis with babies, a spotted cat, and a plecostumus in that same tank with my peacocks. None of them are exhibiting these symptoms. The symptoms are a sudden lethargy, cloudy eyes, laying on the bottom of the tank behind the rocks, and shallow breathing. I will try and enclose a picture of my sick sulfur head peacock.


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I thought I would add that I tested the water, and nitrates were at 10, ammonia was 0-0.25, and the fresh water high range pH registered between 8.2-8.3. I don't think it's the water; I just hope I get this figured out before I lose any more fish!