Fenbendazole isn't water soluble (or at least easily soluble)... That's why it's better as a food additive.
Levamisol works great but a bit hard to find since they've been taken off the market. What you can find is stock. I don't think levamisol kills the nematode immediately but rather paralyzes the worms. It would detach from the fish and starve.
When treating for worms (callamanus), it's best to treat twice. Once initially and another a week later to catch all stages of the worm cycle.