Flubendazole has been very benign when I've used it. I think Al reported some issues when using with discus, but I haven't noticed any issues with shrimp or catfish. It kills hydra, planaria and expensive snails though. Cheap snails like ramshorn/mts/bladder don't mind (of course).
It has no effect on bacteria or plants. I followed the instructions on inkmkr.com. He sells the best (only?) stuff in the US.
It won't kill tapeworms though; you need prazi for that.
Edit: Forgot to mention, you can't just dump the stuff in the tank. I've had the best luck using a small, empty water bottle. Add the powder, fill it about half-full and shake it hard for 5 minutes or so.