Thanks for the reply.

The fish with fin rot is one of my (4) discus. One of its pectoral
fins is ragged and slowly seems to be wasting away, and it's a bit
whitish on the edges.

The water quality is very good. 0 Ammonia and Nitrites, of course, and
Nitrates 20 ppm. The temperature is at 86, and the GH is 2-3
degrees. (I do water changes daily).

The fish seems healthy otherwise (is eating, etc). The only thing
which happened recently out of the ordinary was a treatment of internal
parasites in one of the others. (They completely recovered in 2
days). I treated with Jungle "Parasite Clear" tabs, since I couldn't
find any plain metronidazole in my area (I've ordered some to be ready
in the future).

Like I said, right now he seems healthy except for the fin rot, which
he's had for probably 5-6 days. It's claimed perhaps 15% of the fin,
or maybe a bit more. He also seems to avoid using the fin sometimes.

What is mercurichrome and where do I get it?
I've often heard of "dabbing" affected areas. What's the best way to
do something like that?

Thanks very much for your help,
Wes