This article is from the Aquaria: Disease, Algae and Snails FAQ, by Elaine Thompson, Thomas Sasala and George Booth
Fishes' fins turn whitish and die back. Fin rot often follows damage
or injury. It can also be caused by poor water quality.
Remedy: First, fix the water and remove any fin-nipping fish. Change
some water (25% is good) and add 1 tsp/gallon salt to promote healing.
If bad water quality or an aggressive tankmate was the problem, that
should be adequate. Healing will begin within a couple of days.
If it worsens, decide first whether it's fungal or bacterial. Fungal
finrot looks like clumps of cotton on the fins and usually follows
injury. It is commonly seen in African cichlids or fish that have
injured themselves against decorations. Bacterial finrot is whitish,
but not cottony (unless it's columnaris), and can be contagious. The
fish then need to be removed from the tank and medicated.
Fungus: For fish large enough to handle, catch the fish, and dab
malachite green directly on the fungus with a Q-tip. This is extremely
effective. Repeat treatments may be necessary.
For small fish, a commercial fungicide such as Maroxy may work. For
severe infestations, try a bath in methylene blue (enough so you can
barely see the fish) until the fungus turns blue or for 20 min. If you
add methylene blue directly to a tank, you will kill plants and trash
your biological filter.
Bacterial: Antibiotic treatment in a quarantine tank. This is
stressful for the fish, and doesn't always work, so be sure of what
you are doing before you attempt it. If the fish is still eating, the
best bet is an antibiotic food. Tetra makes one that works well --
just buy the one for bacterial diseases and follow the directions on
If the fish is not eating, a bath treatment is necessary. A
combination of Kaynamycin and Furanace usually works, especially for
Columnaris. Again, treat in a separate tank and aerate heavily.