Len, for what its worth, it may be a issue of toxins emitted from dying flatowrms. I used it on my display some years ago (300 gallon system with about 12 fish) to combat a serious flatworm infestation. Prior to dosing I spent two weeks siphoning flatworms out - I used a narrow gauge rigid tube attached to a flexible pvc hose , I started a siphon and had the drain end of the hose running into a filter sock in my sump so that caught all the siphoned flatworms.
Siphoning every other day or so then a couple days after most visible flatworms were gone before dosing seemed to work. No fish losses but I did use several airstones and did a 30 percent water change and ran about a pound of carbon through the system once the treatment was complete. There were still large numbers of dead and dying flatworms that crawled out of the rockwork and I can only imagine how bad it would be if I hadn't decreased their number over the preceding two weeks.
The only 'delicate' fish I can recall was a powder brown tang and , sunburst anthias and a magnificent fox face (though in my experience most rabbit fish are tough as nails and can even survive a otherwise total tank crash).