Steve,
Your response is very well thought out. The coloration appears to be a bit blotchy and looks like a little red hemorrhaging back towards the tail fin. In one of the photos it looks like the scales may be lifting a bit. The scales and discoloration may just be distortions in the photos.
Af Hall
Can you see any discoloration or raised scales on the fish when you look at it in person? Since the fish is eating and behaving normally I have my doubts that it is a systemic (the entire system of the fish) bacterial infection. The white in the lesion could be dead white blood cells. There is also a possibility of an imbedded parasite. I don’t see symptoms that look like brooklynella or uronema. The white feces and bump may be unrelated. This is a tough one to diagnose, but I tend to think a bacterial infection. I am not sure that it is vibrio and you might see some dark discoloration rather than the white bumps. It is impossible to identify the type of bacteria involved without a very powerful microscope.
I would suggest treating the fish in quarantine with Maracyn-Two, Furanase or Nitrofurazone (Furacyn). I would also reduce the salinity. The antibiotics will work a bit better in reduced salinity and if the bump happens to be a large embedded parasite then it can leave a large hole when it exits the fish. Reducing the salinity will help the fish maintain osmotic balance when it has a large wound. Stringy white poop in clowns can be caused by bacterial infection or intestinal worms, but it can also be caused by an improper diet. What are you feeding the fish? Do you store all foods including flake and freeze-dried in the freezer? A bacterial infection is more of an immediate threat than internal worms so I would treat with an antibiotic, reduce the salinity, have some Pro Tech Coat Marine or StressGuard handy to serve as a temporary mucus layer in the event of a large wound, and soak the foods in garlic
I would say that the bottom line is that Steve and I are most concerned about bacterial infection at this point.
Cheers,
Terry B