Sores, unless treated early, are liable to develop into fungus. Fish can wound themselves by knocking against sharp pieces of rock or stones when in a panic, or they could be nipped by a larger specimen.
The very best treatment would be to dab the wounds with a little piece of cotton wool soaked in 2 per cent mercurochrome, or by pouring a strong answer of permanganate of potash directly into the wound, and then washing off with fresh water.
The fish obviously requirements to be netted prior to either of the above treatments could be carried out. Keep in mind to deal with the fish carefully and to protect the gills by covering them lightly with a piece of soft material soaked in water.