This is classic cat strike.
scales are removed and there are deep lacerations where the cat has tried to
hold the fish.
Most cats play with fish, as you see in the picture,
very few eat them.
The best treatment to start with is pond salt at 1/oz
per gallon, air stone, room temperature and paint with iodine.
early, the fish should make a full recovery, provided the wounds are not into
the abdominal cavity, mouth, eyes or gills. If it's the latter then destroy the
fish with clove oil as recovery can be as low as