Time for an update ... after a few weeks boating in the PNW. So .. in the end, I lost about 10 fish and for some time, I was resigned to the fact that I would lose them all - if I didn't tear the tanks down to catch all the fish and quarantine them. At it's worst, the two yellow tangs looked terrible. One was so skinny you could see his bones and the other had dark patches on the sides. Of course, I lost both purple tangs and two of the 5 clowns not to mention several other beautiful fish. When I left for the last boating trip about a week ago, I was sure I'd lose another clown but the rest of the fish were back to eating so I left instructions for an aggressive feeding regime. To my surprise, when I got back, all indications of the disease (which I believe was marine velvet) were gone from the clown and no other fish showed any sign of sickness. At this point, all fish are feeding well and there is no sign of disease.
In summary, I did treat with Prazipro but if anything, that seemed to accelerate the disease. The only other thing I did was do a couple of relatively fast water changes (about 15%) with new ocean water that I collect locally. My tanks run on 100% ocean water that I add salt to to bring the salinity up.
One other point of note, while I was away, the caretaker mistakenly ran the salinity up to 0.030 when I normally keep it at 0.025.
Comments are most welcome folks.