Herbtana doesn't even claim to ELIMINATE parasites; just reduce their numbers. As long as any of the protozoan parasites are in a system, the fish are vulnerable. Any stressor can lower a fish's immune response and the ich become prevelant.
I wish I could give you a 1st hand opinion, but I've never used the stuff. However, in the considerable time I've spent in the disease forum, I've never heard of it successfully doing anything. These products come and go; if one did work well, news would be all over the place. It isn't. There is simply no way to eliminate any protozoan parasites (ich, velvet, brook, et al. ) in a reef tank.