Rule of thumb: Never mix different clowns. Its a hit or miss, but mostly a miss. "Black ich" isnt a clown disease, it pretty much only affect tangs, its a tang disease. A pic would help, but i am about 95% sure its not ick or black ick or any of those. When clowns are bullied around and kicked out of the anemone, they seek refugee on anything, including zoanthids. Zoos sting clowns and leave black marks on them, like a burn.