This article is from the Aquaria: Disease, Algae and Snails FAQ, by Elaine Thompson, Thomas Sasala and George Booth
Cichlids and other "scrappy" fish may sustain injuries that are
severe enough to draw blood from fighting. Other fish may run into
tank decorations, walls, or rocks.
Larger fish can be netted and their injuries dabbed with mercurochrome
(available at drug stores) or Betadine (iodine-based antibiotic also
available at drug stores) to help prevent infection. Be sure to keep
these chemicals off of the gills and eyes. For really small fish, put
the affected fish in dilute methylene blue (pale blue) and 1
tsp/gallon salt in a separate tank. If you want to keep the fish in
the main tank just add salt, as methylene blue will trash your
Watch the fish to be sure injuries are healing cleanly, and repeat the
mercurochrome dosage if necessary. If finrot or fungus sets in, see
the above section on finrot.