When I was doing my internship at the local aquarium we treated a few incoming fish with Metronidazole mixed into their food. Generally worked well, if they would eat it. Apparently it doesn't taste very good to the fish. This was used to treat internal parasites in local fish we caught.
I don't know if this is an effective treatment for SW fish or not. The biggest problem we ran into was actually getting the fish to eat the stuff.
I'm not an aquatic vet, so don't know to much about it, I just dosed as prescribed
Mike