If iit IS brook, then I'm not sure how much you can do. I've not heard of anyone here successfully treating a fish with brook. Not that it hasn't been done, but I haven't heard of it in this group.
Your fish may survive though, especially if some are not showing sypmtoms. Both times I lost bannerfish, the other 5 fish in the tank (clowns, goby, blennie, and maj. angel) were unaffected. They never suffered any syptoms. I've heard that fish succumb to brook when very stressed and unhealthy, so I'm guessing that the established fish in my tank were healthy enough and the tank healthy enough to allow them to escape unscathed.
I think the treatment is with formalin, so you could either try taking the fish out and treating them, or you could try to keep them fed, and see if they survive. Chances are they'll be dead in a day though.

Then you can let the tank lie fallow for a month, and let it completely cycle. Next time, put the fish in gradually, and hope for better luck. Sorry you had to start out this way.
