[DJRansome]

I would isolate him in a hospital tank with nitrates < 20-ppm.

What are the dimensions of your tank? What fish do you have stocked: counts of each species and genders.

 

[DJRansome]

OK so the borleyi is too big for a 55G.

Mbuna like zebras don't mix well with haps and peacocks. Try removing the cobalt permanently.

Meanwhile post a pic of your unknown African in Unidentifed. If he is a mbuna, you may want to remove him permanently as well.