I've had a bad experience with Alkylamine Salts of Endothall (Hydrothol 191). It does kill fish, and I lost fish in my hatchery pond. They started floating within a few hours.
If the water is void of fish, or shallow enough that small fish aren't swimming in the problematic area, then it does work well. I did use up my bag of Hydrothol, but I was very selective of where I used it.
