I feel for you dealing with sick fish.
Flubendazole, levaisole hcl and metronidazole all can be dosed via the water column and work well treating parasites, often much better than food based fenbendazole treatments. Flubendazole and levaisole hcl are both used here during entry quarantine and annually to prevent trouble in my systems. Combined the treatments eliminate the vast majority parasite problems. While any medication should be used with caution healthy animals rarely react badly. I think we need to realize fish are usually chronic before you see secondary symptoms. Losses here have been largest during the first dosing.
Most of the time treating sick fish is based on guesswork then broad spectrum 'sledgehammer' treatments are the only choice for most and reading this thread it's no different, not really sure. Once fish are showing external symptoms internal damage is serious. Losses can easily be those fish we 'see' as healthy that in reality are not.
couldnt agree more