Mouth or Tail Rot is quite often caused by flukes. If not treated aggressively and quickly the mouth will deteriorate to the point the fish will not be able to eat. Every passing day will see the mouth grow progressively worse. The quicker one acts the less likely the mouth will not be permanently deformed. Many fish have deformed mouths and it the mouth still opens and closes properly and the fish can eat, the fish will live a normal life. Once the bone structure in the mouth is damaged, the fish is history.