Thanks again for the prompt response!
The neosporin is a good tip. And thanks for telling me how to "dab" it
on.

Looking at the discus again today, it seems most of the "whitishness"
is gone from the fin. I'm not sure if it's still degrading.... Also,
15% was a bit high---it looks more like 10% to me now. In any case,
now I'm thinking of "wait-and-see"-ing, hoping it will start to grow
back on its own. Do you think I'm correct in my assumption that as
long as his behavior is normal otherwise, I'm not risking losing
valuable time to try to treat the problem more aggressively? He's
still eating very well, and in general looks okay, other than the fins.

I do have another tank (20 gallon) that I can use for quarantining the
fish, but, as you suggested, I'm very reluctant to do so with the
discus. In the case of the internal parasites, i also figured that
other fish in the tank were likely carriers (so best to treat
everything).

Thanks again,
Wes