External wounds and infections can be treated with a number of fish medicines, such as King British Disease Clear (for aquaria)
or Ulcer & Open Wound Treatment (for ponds). It is important to keep the water quality pristine, otherwise the wound
will become re-infected. It is also important to find out how the fish got the sore / wound in the first place. If it was
a one-off due to handling, then just try to be more careful next time. However, if it was caused by a heron or a bullying
tank (or pond) mate, then you need to assess whether it could happen again. It might be because you have got too many fish
in the pond or the fish have become too big for their aquarium?