HOLE IN THE GILL

The reason coule be Tuberculosis or Aeromonas bacterial infection. The tuberculosis is not treatable, and will not respond. Tuberculosis is even potentially transmissible to people. If however, it's an Aeromonas infection, it may respond to antibiotics, but the hole will not close. Check for, or treat for a concurrent parasite and for best results, ensure optimal water quality, temperature and aeration.