I found a reference to a disease with these symptoms that kills fairly quickly, it mainly effects koi but aslo can show up in other fish, it shows up as one or more raised red pimple(s) that turn into deep ulcers and is caused by either Aremonas or Pseudomonas bacteria, the treatment is raising the temp. to at least 84 degrees F. and feeding with foods laced with oxytetracycline and oxolinic acid as well as treating the water with forma-green.