Thanks all! Sorry I have not been back to update. The two fish are still alive but not as active as the others, they do seem to swim about when it is sunny. I see this as good news since the fish that died earlier this year, only lived about 2 days once they started acting like this. I am wondering if some of the others were eaten, since I never saw any bodies. . .
All of the fish came in at the same time, or are offspring of those fish, but any imaginable creature visits the pond. What ever parasite this is, is surely ubiquitous. With that in mind, I think I need to focus on keeping stress down. If this is the same "bug" that caused problems in late May, I have to assume that all of the fish have it and succumb when their resistance is compromised. If it is something else, I will never be able to stop the relentless invaders.
Thanks to all for the advice, condolences and links. I think we are headed in the right direction. Hopefully I can get them healthy before winter. . .