Re:disease question

Pete:

Thanks for your excellent, thorough and informative advice. Unfortunately, I came home that night to see the male had passed away. The female had gotten to a point where I could see she was struggling, so I humanely put her out of her misery. I had a hard time of it, but I also wanted to prevent the speading of disease to my new charges.
Both horses did not have the spindles left on the dorsal fin. It seems like the whole fin was eaten away to the back of the horse, also the skin on the thicker part of the tails of both animals seemed to be shedding away and look opaque white. As they got sicker, they appeared to have "seizures" and refused to eat.
My concern now is that the whole tank may be infected, although the other 4 seem to be doing just fine: active, eating, fins are clear. How soon would I know if the others have it? Should I treat the whole tank as a precaution? If so, what should I use to treat it? The tank also contains one very small purple lobster and a few mushrooms on live rock with a fair amount of caulerpa. It is filtered with a two-tier canister filter containing activated carbon and has a sponge skimmer. Heater is set at 74 degrees F. I feed frozen mysis enhanced with Vibrance II twice a day with live ghost shrimp as a treat once a month. Is there anything else you need to know to answer the question?

Thanks,

Pat